By Aaron Ross

KINSHASA (Reuters) - Opponents of Congolese President Joseph Kabila went on strike on Tuesday to demand that he step down when his constitutional mandate expires in December.

In the commercial center of the capital Kinshasa, home to 12 million people, rush hour seemed lighter than usual. Many shops were shuttered in the city's surrounding districts, especially opposition strongholds such as Limete.

Police fired teargas to disperse dozens of protesters from opposition leader Etienne Tshisekedi's UDPS party, who threw stones and erected barricades near the party headquarters in Limete, a Reuters witness said.

But the strike appeared to have significantly less uptake than one in February over the same issue. In the eastern city of Goma and southern mining hub of Lubumbashi, where foreign firms have big investments, residents said that activity largely carried on as usual.

"I really regret seeing our Congolese brothers and sisters open their stores," said Samuel Kazadi, a motorbike taxi driver on Kinshasa's Avenue de Commerce, where several shops had their metal doors locked.

But he added: "We are waiting for Dec. 19 ... Kabila won't stay in power. The population will be in the street every day."

Elections were due to be held in November, before Kabila's mandate runs out on Dec. 19, but will be delayed as authorities enroll millions of new voters.

Kabila's opponents accuse him of dragging his feet on holding the election in order to cling to power in Democratic Republic of Congo, a country that has not seen a peaceful change of government since independence from Belgium in 1960.

Western powers are leaning on him to honor the constitution, which limits a president to two terms in office, and step down. They fear political tensions could reignite a regional war in the country's mineral-rich east that killed millions of people between 1996 and 2003.

U.S. State Department spokesman Mark Toner said in a statement that Washington was monitoring the situation and was concerned "the window to reach consensus on an electoral timeline and plan for transition is narrowing".

Toner urged government and opposition leaders to try to advance a national political dialogue that began on Tuesday with Edem Kodjo, a Togolese diplomat and African Union-appointed facilitator, who is working with the parties on a timetable for the vote.

The main opposition alliance, led by Etienne Tshisekedi, is boycotting the talks, which it sees as a delaying tactic. The talks, expected to last until Saturday, are only meant to set a schedule for further talks whose aim will eventually be to set an election timetable.

In an olive branch to the opposition, the government agreed to free several democracy activists on Friday to try to ease tensions, but Tshisekedi called it insufficient.

Some opposition leaders did show up for the opening session, including Tshisekedi's former chief of staff Albert Moleka and Jean-Lucien Bussa, president of the CDER party.

"Our doors remain wide open. Today or tomorrow, they can join us at any time," Kodjo said in his opening remarks.

Kabila took power when his father was assassinated in 2001, then won his first election in 2006.

(Additional reporting by Amedee Mwarabu Kiboko and David Alexander; writing by Tim Cocks; editing by Alison Williams and G Crosse)