Pro-democracy activist Luc Nkulala was killed in a suspicious fire overnight at his home in Goma, in eastern Democratic of Republic of Congo, his opposition Struggle for Change (Lucha) group said Sunday, calling for a swift and transparent enquiry.

"We suspect that the enemies of democracy and peace carried out this act on our comrade even though, so far, we don't know how the fire started," said Ghislain Muhiwa, a senior figure in the movement which is based in Goma.

"Our comrade Luc Nkulula died in a suspicious fire at his house on Saturday night," he told AFP.

Nkulula was one of the founders of Lucha, a pro-democracy movement strongly opposed to President Joseph Kabila remaining in power.

Lucha, in a statement Sunday, called Nkulala a "hero" and "an irreplaceable model always at the forefront of our action."

The group called on the Congolese authorities to organise "a swift, transparent and credible enquiry with international assistance to determine the circumstances of the death".

A campaigning Catholic committee, which has organised three anti-Kabila marches since late 2017, demanded "a truly independent investigation to determine the circumstances" surrounding the activist's death.

Another pro-democracy activist, Rossy Mukendi, was shot dead during one of the marches organised by the group, close to the Catholic Church.

In August 2016 he was the spokesman for Lucha in a meeting with Kabila four months before the end of the president's official mandate.

Elections for DR Congo's head of state are due to take place on December 23 after two postponements that have stoked fears the sprawling, volatile nation could spiral into war.

Kabila, who has been in power since 2001, had been due to step down at the end of 2016 after he reached his two-term constitutional limit.

In May 2016, however, the Constitutional Court ruled that Kabila could remain in office after the expiry of his mandate until new elections are held, but they have been repeatedly delayed.