WASHINGTON- It should have been one of the most straightforward days of the US presidential transition, with Congress certifying Democrat Joe Biden's victory -- a procedure so routine it usually merits little if any news coverage.

Instead the world watched in shock and horror on Wednesday as a mob of Donald Trump supporters, encouraged by the president, stormed the US Capitol in a bid to thwart lawmakers from their constitutional duty.

Here's how a day of unprecedented chaos unfolded in Washington.

Trump speaks

Shortly after 11 am Trump took the stage at a rally at the Ellipse park, south of the White House fence, to address thousands of cheering supporters wearing "Make America Great Again" gear and tell them he would "never concede."

He repeated calls for his loyal Vice President Mike Pence, set to preside over the events in Congress, to refuse to certify Biden's win.

A number of Republican congressmen had already vowed to object to the certification, but they were outnumbered, making Pence -- as Trump and his supporters saw it -- the last line of defense.

"Mike Pence is going to have to come through for us, and if he doesn't,

that will be a sad day for our country," Trump told the crowd, promising he

would "be there with you" as they marched on Congress.

Pence rebuffs the president

But even as he was speaking on stage Pence, who has no authority to do as Trump was asking, issued a statement publicly defying his boss for the first time in four years.

"The Constitution constrains me from claiming unilateral authority to determine which electoral votes should be counted and which should not," Pence said.

Congress begins certifying Biden's win

Moments later Pence launched the Congressional proceedings, with television networks showing extraordinary split screens of the vice president beginning to certify Biden's victory inside the Capitol as the president, still on stage in the park, continued to rail against it.

Before the president finished talking, however, his supporters had begun to drift away -- headed for the Capitol.

Trump supporters storm Capitol

Inside, Arizona had become the first state to object to the certification, forcing a debate. Top Republican senator Mitch McConnell was warning of a democratic "death spiral" if Biden's win was overturned.

The first sign of trouble came as some Congressional offices were evacuated with pro-Trump protesters demonstrating outside. Shortly after, the mob began storming the Capitol itself.

Thousands descended on the building. Photos emerged of Capitol police with guns drawn at the door of the debating chamber as terrified lawmakers took cover under chairs and camouflaged protesters smashed windows, and Washington's mayor ordered a nighttime curfew.

Pence, others call for calm

Trump tweeted a call for the mob to "stay peaceful." Minutes later the first reports of one person being shot and wounded inside the US Capitol emerged. The victim, a woman, died several hours later.

Pence, also on Twitter, issued his own -- much more forceful -- call for the violence to "stop now." Lawmakers from both sides of the aisle as well as foreign leaders watching in horror began following suit.

Biden takes the stage

With the president holding back from forcefully condemning the violence it was his successor, Joe Biden, who was the first to go on television and brand it an "insurrection."

"I call on President Trump to go on national television now to... demand an end to this siege," the veteran Democrat said in Wilmington, Delaware.

"To storm the Capitol, to smash windows, to occupy offices, the floor of the United States Senate... threatening the safety of duly elected officials?

It's not a protest, it's insurrection."

Trump to protesters: 'We love you'

Minutes after Biden spoke, Trump tweeted a video in which he told supporters to "go home" -- but repeated his incendiary claim the election was "stolen" and told them "We love you."

"I know your pain," Trump said in a one-minute video on Twitter. ""We had an election that was stolen from us.

"But you have to go home now," he said. "We have to have peace. We have to have law and order."

Riot police push mob back

But -- as Facebook and Twitter began limiting and removing both Trump's posts and other posts stoking violence -- police in riot gear and backed by the National Guard set up a perimeter around the Capitol building.

Using more tear gas they finally began to push the mob back.

As the 6:00 pm curfew began, thousands were still surrounding the grounds and streets outside the Capitol.

Congress vows to resume

US media quoted the sergeants-at-arms of the House of Representatives and the Senate as saying the Capitol was finally secure about four hours after it first came under attack.

House Speaker Pelosi said the Biden win certification would resume "tonight," once the Capitol was secure.

The senior lawmaker blasted the storming of the Capitol as a "shameful assault" but said "it cannot, however, deter us from our responsibility to validate the election of Joe Biden."

Shortly after 8:00 pm, lawmakers resumed their task.

UN heads 'saddened', 'deeply concerned' by US Capitol violence

General Antonio Guterres and president of the General Assembly Volkan Bozkir both condemned Wednesday's violence at the US Capitol.

Guterres, according to his spokesman Stephane Dujarric, was "saddened" by the breach of the US Capitol building by supporters of outgoing US President Donald Trump.

"In such circumstances, it is important that political leaders impress on their followers the need to refrain from violence, as well as to respect democratic processes and the rule of law," Dujarric said in a statement.

Former Turkish minister Bozkir pointed out the particular relationship between the UN and the United States as the organization's host country.

"Deeply concerned by violence at the Capitol in #WashingtonDC and the interruption of the democratic process in the UN's host country," he tweeted. -AFP