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"I swear to formally assume the national executive powers as acting president," Juan Guaidó told cheering crowds of protesters on Wednesday.
Minutes later, US President Donald Trump recognised the 35-year-old as the country's interim leader.
He declared President Nicolás Maduro's government "illegitimate" and encouraged other countries to back the opposition politician.
But who is Juan Guaidó?
His election as leader of the opposition-held National Assembly has re-energised opposition to Mr Maduro in a country crippled by a severe economic crisis.
But he was relatively unknown until picked as the head of the legislative body just three weeks ago.
Mr Guaidó was born one of seven children in the port city of La Guaira in the state of Vargas.
He was 15 years old when Nicolás Maduro's predecessor, Hugo Chávez, came to power in 1999.
That same year heavy rains caused flash floods which swept through La Guaira, killing tens of thousands of people and destroying large swathes of the city.
Mr Guaidó and his family survived the Vargas tragedy, although people close to him reportedly say the government's ineffective response to the tragedy drove him into politics.
He studied industrial engineering at university, before completing graduate degrees at George Washington University in the US capital and Venezuelan private business school Instituto de Estudios Superiores de Administración.
As a student, Mr Guaidó protested against what he saw as Mr Chavez's efforts to control the media when he did not renew the license of independent broadcaster Radio Caracas Television.
Mr Guaidó was a founding member of centrist political party Popular Will in 2009, along with key opposition leader Leopoldo López.
Mr López, now under house arrest, reportedly acted as a mentor to him.
The pair speak several times a day, according to Bloomberg, despite Mr López's detention.
Mr Guaidó was elected to the National Assembly in 2011 and became a representative for his home state in 2016.
Opposition parties picked him to lead the body as a compromise candidate, and he took up his post on 5 January.
His youth and his background - Vargas is one of Venezuela's poorest states - means the government has struggled to paint him as a member of the country's elite.
State security forces arrested Mr Guaidó just days after his election, with video on Twitter allegedly showing men dragging him from a car.
But he was released from detention hours later and travelled to a rally as scheduled.
Mr Guaidó has called on the army to withdraw their support for Mr Maduro, promising an amnesty for those soldiers who do so.