Venezuela's opposition leader has won the support of more countries in his bid to oust the "dictatorship" of President Nicolas Maduro.
It has sparked violence in the South American country and at least seven people have been killed in clashes between government forces and demonstrators.
US President Donald Trump said Mr Guaido had "courageously spoken" against Mr Maduro, who was described this week as a dictator by Vice President Mike Pence.
Britain, Germany, France, Argentina, Brazil, Canada, Colombia and Chile are among the countries that have also backed Mr Guaido.
A spokesman for Prime Minister Theresa May said on the UK supported "the democratically elected National Assembly with Juan Guaido as its president".
France's President Macron tweeted that he "salutes the courage of hundreds of thousands of Venezuelans marching for their liberty".
Russia, however, has said Mr Maduro is the legitimate president and attempts to remove him are illegal.
Mr Guaido, 35, the new leader of the assembly, announced his dramatic move to cheering supporters in the capital, Caracas - two weeks after Mr Maduro was sworn in for a controversial second term.
After claiming the presidency, Mr Guaido told the crowds: "We know that this will have consequences.
"[Guaido] was surrounded by people screaming 'president, president' as he walked through a rallying point," said Stuart Ramsay, Sky's chief correspondent, who is in Venezuela.
"This will end in violence, I can guarantee that, but it has already been violent."
Last May's election was boycotted by the opposition and Mr Maduro's strongest opponents were blocked from taking part.
Mr Maduro responded to America's backing of his rival by saying he would cut diplomatic relations - giving US officials 72 hours to leave.
"I've decided to break diplomatic and political relations with the imperialist US government," he declared.
Mexico, Bolivia and Turkey have also sided with Mr Maduro.
He said: "We are going to stay in the streets, and stay in battle, for now and forever."
Patrick Duddy, former US ambassador to Venezuela, told Sky News: "It was bad before - it's dreadful now. Inflation last year was over 1,000,000%.
"Oil production plummeted to below 1.2 million barrels per day, it was above three million barrels per day when Chavez took office.
"It's anticipated that millions more will leave the country this year."