Nicolas Maduro was sworn in as acting president of Venezuela after a state funeral for Hugo Chavez.
Mr Maduro, the vice president, was being sworn in at the same military academy complex where the president has been lying in state.
On Tuesday, after announcing Mr Chavez’s death, the government said Mr Maduro would be both acting president and the presidential candidate of the governing party.
The opposition says it will not attend the swearing-in.
Opposition spokesman Angel Medina says Mr Maduro’s ascension is “a violation of the constitutional order”.
Critics believe Venezuela’s 1999 charter stipulates that the speaker of the National Assembly takes power in the event of a presidential death.
Presidents and other dignitaries began arriving by mid-morning for the ceremony, and were still streaming in after the funeral’s scheduled start.
More than 30 heads of state attended, from Iranian president Mahmoud Ahmadinejad to Cuba’s Raul Castro, to the leaders of Mexico, Chile and Brazil.
US Representative Gregory Meeks, a New York Democrat, and former Representative William Delahunt, a Democrat from Massachusetts, represented the United States, which Mr Chavez often portrayed as a great global evil even as he sent the country billions of dollars in oil each year.
The Rev Jesse Jackson tweeted that he would be there, and Hollywood star Sean Penn also said he would attend.
Mr Maduro has said Mr Chavez’s embalmed body will be permanently displayed in a glass casket so that “his people will always have him”.
He said the remains will be put on permanent display at the Museum of the Revolution, close to the presidential palace where Mr Chavez ruled for 14 years.
Tens of thousands have already filed past his glass-topped casket at a military academy following a seven-hour procession on Tuesday which took his body from the hospital where he died.