More than a dozen people have been killed and hundreds injured in clashes between opposing groups of protesters in New Delhi, hospital officials said on Tuesday, as riots overshadowed US President Donald Trump's first official visit to India.

The clashes, the worst in the capital since unrest over a new citizenship law began in December, started at the weekend but turned deadly on Monday.

Violence erupted again in multiple areas of northeast Delhi on Tuesday, just miles away from where Trump and Indian Prime Minister Narendra Modi met for talks.

After the talks, Trump told reporters that he had heard about the violence, but did not discuss it with Modi.

He later declined to comment on the citizenship law. "I don't want to discuss that. I want to leave that to India and hopefully they're going to make the right decision for the people," Trump said.

According to the Scroll.in, angry mobs threw stones and petrol bombs at Muslim homes in Kabir Nagar, near Maujpur. Journalists were also attacked and many were forced to delete pictures and videos from their phones, the publication added.

The report added that Delhi Chief Minister Arvind Kejriwal had said he had a "positive meeting" with Home Minister Amit Shah regarding the riots in the Indian capital. Speaking to the media, he maintained that police were outnumbered and were unable to take action as they did not have orders to do so, the publication said.

According to The Wire, stone-pelting and arson have continued in the Ghonda area, close to Maujpur.

Across the locality, The Wire saw broken and burnt vehicles, burnt shops and vehicles still on fire. There were some police standing at the edge of the Hindu-majority area, but did not intervene when vehicles were burning, the publication said.

In the Bhajanpur area in the city's northeast, black smoke rose into the sky after Hindu protesters set fruit and vegetable shops and a Muslim shrine on fire, witnesses said.

Meanwhile, a mosque in Ashok Nagar neighbourhood was set on fire late on Tuesday afternoon, The Wire reported. It said a mob shouting “Jai Shri Ram” and “Hinduon ka Hindustan” paraded around the mosque, and a Hanuman flag was placed on the mosque's minaret. Shops near the mosque were looted by the mob.

Commenting on the development, President Arif Alvi in a tweet termed the vandalising of the mosque a "disgraceful act". "It seems to be a reminder to Muslims of the Babri Masjid episode. I think secular forces within India should rise against such barbaric actions," he wrote.

The clashes erupted in a northeastern district of the city between thousands of people demonstrating for and against the new citizenship law.

Eyewitnesses said the violence continued overnight, and several shops were broken. “There was stone pelting in the area early this morning also,” a resident was quoted as saying.

Tensions in parts of the city remained high on Tuesday with schools remaining shut in some areas amid news reports of fresh clashes. At least five metro stations in the city were closed.

Deputy Chief Minister Manish Sisodia tweeted that schools in the capital's northeast would be shut on Tuesday and exams postponed.

An official at Guru Teg Bahadur Hospital in New Delhi said 13 people had died there and more than 150 injured had been treated at the hospital, many from bullet wounds.

An official at the Al-Hind Hospital in New Delhi said two men had died and over 200 injured had been treated there.

A fire department official said his teams were responding to more than a dozen calls over arson attacks, despite emergency measures prohibiting any gatherings in the violence-hit areas.

“We have sought police protection as our vehicles are being blocked from entering the affected areas. The situation is very grim,” Delhi Fire Department Director Atul Garg said.

One fire department vehicle was torched by protesters on Monday and a small number of firemen were injured, Garg added.

The joint commissioner of police, Alok Kumar, said around 3,500 police and paramilitary officers had been deployed.

Police fired teargas and pellets to scatter demonstrators protesting against the citizenship law in the Jafrabad neighbourhood of northeast Delhi on Tuesday. A young man suffered a pellet wound to his thigh, while another sustained injuries to his eyes, leaving his face bloodied.

Chaos ensues

The violence on Monday began just as Trump arrived on his maiden visit to India.

At a press conference early on Tuesday, Delhi Chief Minister Arvind Kejriwal appealed for people to maintain peace.

"Whatever problems people have can be resolved peacefully," he said. "Violence will not help find a solution."

India's capital has been a hotbed of protests against the new Citizenship Amendment Act (CAA), which eases the path of non-Muslims from three neighbouring Muslim-dominated countries to gain Indian citizenship.

This has led to accusations that Modi and his Hindu nationalist Bharatiya Janata Party (BJP) are undermining India's secular traditions.

The BJP denies any bias against India's more than 180 million-strong Muslim minority, but objectors have been holding protests and camping out in parts of New Delhi for two months.

Reuters reporters saw multiple vehicles set on fire, metal barricades torn down and thick smoke billowing as supporters of the new law clashed with opponents. A small contingent of police was vastly outnumbered.

'Go ahead and throw stones'

The clashes stretched over a nearly kilometre-long road and lasted from early afternoon until well into the evening, leaving at least half a dozen people injured.

Police on Monday imposed an emergency law prohibiting any gatherings in the violence-hit areas. The protesters appeared to be divided along religious lines, and some repeatedly hailed Hindu god Ram while pelting stones at the predominantly Muslim group opposed to the new law.

A Reuters witness saw at least one Muslim man and a burqa-clad woman were beaten by dozens of pro-law protesters with sticks and iron rods.

Earlier on Monday, local politician Yogendra Yadav described the violence as "communal" and urged police to intervene.

But for several hours, Reuters witnesses saw the outnumbered policemen mostly stand beside those backing the citizenship law, a majority Hindu group, and do little to quell the violence.

The police also stood by as a mob vandalised a store with a Muslim name, pulling out vehicles and setting them alight.

"Go ahead and throw stones," one policeman shouted to protesters backing the law, during one of the running battles.

On side streets, young boys drained fuel from motorcycles to make petrol bombs, and protesters pulled up paving stones and flung construction material at opponents.

"We are in support of the CAA. If they want to protest, they should go somewhere else," said a protester who gave his name as Amit.

India's junior home minister G Kishan Reddy said the violence was a "conspiracy to shame India globally" during Trump's visit.

"We have deployed additional forces, and we will do everything to bring law and order under control," he told Reuters partner ANI.

National BJP spokesperson Gopal Krishna Agarwal echoed similar thoughts. “The timing of these riots creates suspicion about them being politically motivated. For over 60 days, people were protesting in Shaheen Bagh without any significant outcome. Our government believes that dissent has a place in democracy and they have the right to protest within the established process.

“But indulging in such violence is not the way. Surely, when the head of a nation is in India and such violence is taking place, there is definitely a group, a politicial design behind it. This is being done to tarnish the image of India,” he said, according to The Print.

Ahead of Trump's arrival in India, a senior US administration official had said Trump would raise the issue of religious freedom in India during his meetings with Modi.

Addressing a gathering of more than 100,000 people in Gujarat, however, Trump made no mention of the protests enveloping India over its Citizenship Amendment Act.

At the rally in Ahmedabad, Trump praised India’s history of religious tolerance, and “many faiths worshipping side-by-side”.

"India is a country that proudly embraces freedom, liberty, individual rights, the rule of law, and the dignity of every human being," he said.

'The world has been shown the truth'

In Pakistan, Special Assistant to the Prime Minister on Information and Broadcasting Dr Firdous Ashiq Awan said that the violent clashes on India's citizenship law, in President Trump's presence, have shown the world the truth.

"Has the world [now] seen who suffers from mental illnesses like prejudice, extremism and warmongering?" Awan said in a tweet.