Supporters of Etienne Tshisekedi, the Democratic Republic of Congo’s veteran opposition leader, have gathered in the capital, Kinshasa, to express their shock at his death.
A prominent opponent of successive leaders, he was due to head a transitional council under a deal for President Joseph Kabila to step down.
The 84-year-old died in Belgium where he went last week for medical checks.
The information minister said he would be given a state funeral.
Mr Tshisekedi returned to Kinshasa last July to a hero’s welcome after two years in Brussels for medical treatment.
Correspondents say his death comes at a sensitive time for DR Congo. There were fierce clashes last year when it was announced that President Kabila would stay in power until April 2018.
Witnesses say as word of his death spread in Kinshasa on Wednesday evening, clashes broke out between a small group of his supporters and police, who fired teargas.
Mourners who gathered at his son’s house in Kinshasa said they were concerned about the future.
“This man sacrificed his life, his youth for us all. This man made us open our eyes. He was our icon. This man was an icon for Africa. He was great. We lost a great man,” one woman told the BBC.
Another said his supporters saw him as incorruptible: “He was an historic opponent. [Nelson] Mandela was the best and Tshisekedi comes after.”
According to the Reuters news agency, his son, Felix, is tipped to be named prime minister in a forthcoming power-sharing government which will see Mr Kabila leave office.
Mr Tshisekedi served as a minister under autocratic ruler Mobutu Sese Seko in the country then known as Zaire before helping to set up the UDPS.
During the 1990s he was named prime minister four times but did not remain in the position after clashing with Mobutu, who was eventually forced out in 1997.
His activism meant he remained popular but in recent years he had been suffering ill health.