Etienne Tshisekedi, the main opposition leader of the Democratic Republic of Congo, has died in Brussels aged 84, according to diplomatic sources.
Tshisekedi was set to take the top post in a transitional council agreed in December under a deal that would pave the way for President Joseph Kabila to leave power in 2017 and refrain from running for a third term.
His death deprives the opposition of its principal figurehead as talks over implementation of the December accord falter.
His son, Felix, is tipped to be named prime minister in a forthcoming power-sharing government.
The Union for Democracy and Social Progress (UDPS) party said he went to Brussels last week for a medical check-up.
Tshisekedi stood up to Mobutu Sese Seko, who ruled the country then known as Zaire, for decades before being overthrown by Rwanda, Uganda and other forces.
A pivotal figure
He was also the most prominent civilian opponent of Laurent Kabila, who took power in 1997, and his son, President Joseph Kabila, who has ruled since 2001.
As such, he was a pivotal figure in DRC, a country whose history has been marked by foreign intervention, civil war, coups and authoritarian rule.
Tshisekedi served as a minister under Mobutu before helping to found the Union for Democracy and Social Progress party (UDPS), the first organised opposition platform in Zaire, in 1982.
He was named prime minister four times in the 1990s as Mobutu contended with pro-democratic currents in the country, but Tshisekedi never lasted more than a few months as he repeatedly clashed with the autocrat.
He finished runner-up to Kabila in the 2011 presidential election. International observers said the vote was marred by fraud and Tshisekedi's supporters have referred to him ever since as the "elected president".
Source: Reuters news agency