Goma - Congo's main opposition leaders and rights groups called on Monday for an investigation into the death of Luk Nkulala, a leading pro-democracy activist and critic of President Joseph Kabila.

Nkulala was killed in a fire in his home in the eastern city of Goma on Sunday, and although authorities say there is no evidence of foul play, many believe his death was suspicious.

In a Monday tweet, opposition leader Felix Tshisekedi alleged that Nkulala had been "cowardly assassinated," adding that the "main suspect is the power in Kinshasa."

Authorities in North Kivu province, where Nkulala lived, said they are investigating but warned against "speculation."

"There is no evidence it was arson," said Roger Rachidi Tumbula, a spokesman for North Kivu province's governor. "Let's not speculate."

Calls are mounting for an independent probe, with another opposition leader, Moise Katumbi, tweeting that "light must be shed" on Nkulala's death.

Ida Sawyer, Central Africa director for Human Rights Watch, also called for an investigation.

"In recent years (Nkulala) called for Kabila to step down in accordance with the constitution and allow for credible, transparent elections," Sawyer said in a statement.

"Congolese security forces arrested Luc often during these peaceful protests.... Even after days in jail and police beatings, Luc never appeared discouraged," she added.

The 33-year-old was a founder of LUCHA, a civil society organisation.

Kabila was supposed to leave office at the end of his term in 2016, but he refused to do so and has failed to hold elections in the years since. He has also refused to say if he intends to run again in elections planned for December.