The US Congress certified on Thursday that Democrat Joe Biden and his running mate Kamala Harris have won the 2020 presidential election, but the certification followed a mob attack on the Capitol building that resulted in the deaths of four people.

The invasion also forced lawmakers to abandon the premises and they returned only after Washington’s Mayor Muriel Bowser imposed a curfew and declared a 15-day emergency in the US capital.

Washington Police Chief Robert J Contee told a news briefing late on Wednesday that one woman died in a local hospital after she was hit by a bullet when a police officer discharged his gun. Three others — two men and a woman — died of medical emergencies as the mob blocked all roads. Fourteen police officers were also injured in mob attacks.

Police have arrested 52 people, most of them for curfew violations. They also discovered two pipe-bombs at the headquarters of the Republican and Democratic parties.

Chief Contee said police also seized weapons, including pistols, guns and explosive devices, from the crowd. He said the troubles started at 2:45pm when crowds stormed security barricades at the Capitol building and continued for several hours.

In pictures: Chaos, violence, mockery as pro-Trump mob occupies Congress

After police removed protesters from the Capitol Hill area, the lawmakers resumed the process of certification.

Both the Senate and the House of Representatives voted twice on Wednesday night, rejecting efforts to overturn election results in the states of Arizona and Pennsylvania. Vice President Mike Pence, who is also the president of the Senate, presided over all joint sessions.

Republican lawmakers had planned to seek reversal of electoral votes in six swing states — Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin.

Even after the attack, dozens of Republican House members pushed forward their plan to undo election results but after Wednesday’s mob storming, senators refused to co-sign all but two objection certificates, as required.

Before the mob raid, 14 Republican senators had offered to sponsor the notices but when the debate resumed, they only signed objections from Arizona and Pennsylvania, which led to two separate debates and votes.

Earlier in the night, 93 senators voted to reject the Arizona objection, only six voted for the motion. Later, the Senate also rejected the attempt to overturn the will of Pennsylvania voters by 92 to 7 votes.

In the 435-member House, 282 voted to reject the objection while 138 voted for it. In the earlier vote, 302 voted to reject the objection to Arizona’s election while 82 voted for it. No Democrat voted for the two objections while dozens of Republicans voted against it.

Some House members did not participate in the two votes. Republicans are a majority in the 100-member Senate while Democrats control the House. The House held its second voting at 3:10am.

The late-night joint session began at 3:27am and completed its proceedings within half an hour. No senator signed the objection certificate for the sixth disputed state of Wisconsin when the session began.

The meeting concluded that Biden and Harris had received 306 of the 538 electoral votes while Trump and Pence had received 232 votes.

Pence declared Biden and Harris the winners of the 2020 election, adding that they should take the oath of their offices on Jan 20, as required.

The session ended with a prayer by the chaplain of the US Senate who also regretted the loss of life in Wednesday’s violence at Capitol Hill.

"When it's over, it's over. Joe Biden and Kamala Harris are lawfully elected and will become the president and vice president of the United States," said Republican Senator Lindsay Graham while commenting on Thursday’s events.

Although the votes paved the way for Biden to take the oath as the 46th president on Jan 20, the mob attack overshadowed the certification.

"What happened here today was an insurrection incited by the president of the United States,” said Mitt Romney, a Republican from Utah.

Mitch McConnell, the Republican majority leader in the Senate, called the storming an attack on democracy. "They tried to disrupt our democracy. They failed. They failed,” he said during the debate.

When the joint session began, Trump was addressing a large crowd of his supporters at the White House, urging them stop the process that would bring in a new president on Jan 20.

“You have to get the people to fight. And if they do not fight, we have to primary the hell out of the ones that don’t fight,” he said. “You primary them, we are going to let you know who they are. I can already tell you, frankly.”

Trump encouraged the raid after it became clear that the people he hoped would support his bid to retain the White House — particularly Pence and McConnell — would not support his move as both vowed to follow the constitution and not his instructions.

Read: 'Disgraceful scenes':World leaders condemn 'assault on democracy' at US Capitol

"It is my considered judgment that my oath to support and defend the constitution constrains me from claiming unilateral authority to determine which electoral votes should be counted and which should not," Pence wrote in a letter to members of Congress before he gaveled in the joint session.

“The United States Senate will not be intimidated," McConnell said on the Senate floor when senators returned to the chamber under police protection. "We will not be kept out of this chamber by thugs, mobs, or threats."

Pence directly addressed the demonstrators when he reopened the evening session. "You did not win,” he told them.

A California Democrat, Rep Jimmy Gomez, suggested charging Trump with treason. "I think Donald Trump probably should be brought up on treason for something like this," he told reporters. "This is how a coup is started. And this is how democracy dies."

Congress now resumes its joint session to consider other objections.