By Benoit Nyemba

KINSHASA (Reuters) - Violence broke out on Thursday between supporters of rival Congolese opposition parties, exposing deep divisions among President Joseph Kabila's adversaries over whether to engage in talks about a delayed presidential election.

The talks between the government, opposition and civic leaders opened later on Thursday. Authorities said last month that the poll, set for November, could not be held before next July as they enroll millions of new voters.

Kabila's opponents accuse him of stalling the vote to hang onto power, a charge he denies. Most of the main opposition parties are boycotting the talks but some prominent figures have agreed to participate, saying they will use the forum to insist on his departure this year.

About 100 supporters of parties opposed to the talks threw rocks and burned tyres in front of the headquarters of the UNC and ATD parties, whose presidents have agreed to participate.

Some chanted, "We're going to burn the headquarters of Kamerhe. Kamerhe is a traitor," in reference to UNC president Vital Kamerhe, who is leading the opposition delegation at the talks.

Police fired tear gas to disperse the crowds and made about 20 arrests, a Reuters witness said.

The police said in a statement that they later arrested 85 people outside a hotel where they were trying to "sabotage" the opening of the talks. In fact, the demonstrators were mistaken, as the opening ceremony was actually being held at a venue 9 km (5.6 miles) away.

Addressing the delegates there, Kamerhe appealed to other factions to join the negotiations, saying the Congolese people would be watching closely to see "who is the first to violate the constitution".

Kabila won disputed elections in 2006 and 2011 after succeeding his assassinated father, Laurent, in 2001. The constitution limits him to two elected terms but the country's highest court says he can remain beyond the end of his mandate in December until the election takes place.

The opposition in Democratic Republic of Congo has long been divided. International powers fear an outbreak of violence in a country that has never experienced a peaceful transition of power, and where millions died in wars between 1996 and 2003.

(Additional reporting and writing by Aaron Ross; Editing by Mark Trevelyan)