DHAKA: Authorities have rounded up about 1,600 criminal suspects, including a few dozen believed to be Islamist radicals, in a nationwide crackdown aimed at halting a wave of brutal attacks on minorities and activists in Bangladesh, police said Saturday.

The attacks — including on two Hindus in the last week — have alarmed the international community and raised questions about whether Prime Minister Sheikh Hasina's secular government can maintain security for minorities in the Sunni Muslim-majority country.

Police and paramilitary soldiers fanned out across the country Thursday night, raiding suspected militant hideouts and detaining about 1,600 people by Friday night, police said. The majority of those detained, however, are described as petty criminals.

Only 37 of them are suspected to be radical Islamist militants, according to police spokesman Kamrul Islam. Those include three charged with alleged membership in the banned militant outfit Jama'atul Mujahideen Bangladesh.

None of those arrested is believed to be a high-level operator who might have organised or ordered attacks, police said. All the detainees are being held in jail.

Hasina's government has faced criticism for failing to prosecute suspects for at least 18 killings carried out over the past two years. Victims include atheist bloggers, foreign aid workers, university professors, gay rights activists and religious minorities including Hindus, Christians and Shia Muslims.

Hasina had announced the anti-militancy campaign after the wife of a police superintendent was shot and stabbed to death on June 5 as she was waiting with her son at a bus stop.

The victim had been an ardent campaigner against Islamist militants, and her murder stunned the country's establishment, many of whom considered the victim as one of their own.

Speaking to Parliament on Wednesday, Hasina vowed to root out radicals bent on spreading terror and violence in a bid to restore the country to Islamic rule.

"If they think they could turn Bangladesh upside down, they are wrong," she said. "They will be exposed to justice in the soil of Bangladesh and their patrons will also not be spared."

The attacks have followed a pattern: A group of young men wielding knives or machetes approach their victim as his or her guard is down, perhaps while strolling down the street or relaxing at home. The attackers spew hateful language, then hack and stab at the victim before disappearing, often without a trace.

Many victims are killed with a machete blow to the back of the neck. Authorities have arrested some suspects in some of the 18 attacks, mostly low-level operatives accused of following orders to carry out attacks, but none has been prosecuted.

Police have said they are waiting until investigations are complete before taking any suspects to court.

Amnesty International has criticised the government for inaction, saying it is creating a culture of impunity. It also said authorities are failing to address increasing numbers of reports of people receiving threats.

"The brazen announcement by violent groups that they will continue targeting those they perceive as 'insulting Islam' should shake the Bangladeshi authorities out of their complacency," Champa Patel, the right's group's director in South Asia, said in a statement.

"Ignoring the problem is not a solution. The authorities must categorically condemn these killings, carry out a prompt, thorough, impartial and transparent investigation, deliver justice for the victims, hold the perpetrators accountable, and protect those still under threat."

Nearly all the attacks have been claimed by transnational Islamist extremist groups, including the militant Islamic State group and various affiliates of Al Qaeda.

The killing Friday morning of a Hindu ashram worker in northern Bangladesh was also claimed by the IS, according to the SITE Intelligence Group, which monitors jihadist activity online and cited the Amaq News Agency.

Hasina's government, however, says transnational terror groups have no presence in the South Asian nation of 160 million. It blames the attacks on domestic groups aligned with political opposition parties, though it has presented no evidence of such a campaign and the opposition denies the allegations.

On Friday, the opposition BNP party said it was worried the government campaign against extremists would lead to efforts to suppress opposition parties.

"The crackdown is a strategy which the government earlier used to suppress the people's movement. We fear that they will again oppress the opposition in the name of conducting a crackdown," BNP Secretary General Mirza Fakhrul Islam Alamgir said.