The protesters who sought to take over the US Capitol building on Wednesday generated anger and concern about the nation's democracy across the country and among world leaders.

"Peaceful protest is an important mechanism of our democracy but what we are witnessing in our nation's Capitol building is reprehensible and an outright assault to our democracy and Democratic institutions," California Governor Gavin Newsom said.

"Because of today's events incited by the President, I had to ask the Illinois State Police and other enforcement resources, engaged in important life-saving missions (COVID-19 vaccination), to redeploy to heighten their presence at government buildings and the Capitol in Springfield," said Illinois Governor JB Pritzker.

"This violence is the natural conclusion of Donald Trump's efforts to unleash the forces of hate and divide in our nation. It must end," he added.

Republican leaders in Congress also issued sharp condemnations of the riot, with some blaming Trump for the violence.

"It is a riot masquerading as a protest," said Todd Gilbert, the Virginia House minority leader. Other Republican leaders echoed that.

US Senator Marco Rubio of Florida on Twitter asked Trump to "restore order" in the US capital, saying "there's nothing patriotic about what is occurring on Capitol Hill".

Wednesday's unrest also raised concerns among leaders around the world, and some held Trump personally responsible for it.

"What we are now seeing from Washington is a completely unacceptable attack on democracy in the United States. President Trump is responsible for stopping this. Scary pictures, and amazing that this is the United States," wrote Norwegian Prime Minister Erna Solberg.

Irish Minister of Foreign Affairs Simon Coveney described the chaotic scenes in Washington DC as "shocking" and "deeply sad" and called it "a deliberate assault on Democracy by a sitting President & his supporters".

Leaders and top diplomats in the Netherlands, Iceland, France, Austria, Colombia, Scotland and others also issued similar messages, reminding the US of its role as a model of democracy in the world.

Several Congress members are calling for impeaching Trump for a second time. He was impeached in December 2019, on charges of abuse of power and obstruction of Congress related to his efforts to pressure the president of Ukraine to investigate the Biden family.

"I am drawing up Articles of Impeachment," Representative Ilhan Omar from Minnesota wrote on Twitter.

Ayanna Pressley, a representative from Massachusetts tweeted, "Donald J Trump should immediately be impeached by the House of Representatives and removed from office by the United States Senate as soon as Congress reconvenes.."

Liu Yinmeng in Los Angeles contributed to this story.