(Corrects paragraph nine to make clear Bukavu is in South Kivu, not North Kivu)
By Aaron Ross
KINSHASA, Sept 13 (Reuters) - Democratic Republic of Congo's opposition pledged to escalate its campaign against a possible third term for President Joseph Kabila after police dispersed a protest march with teargas in the capital Kinshasa on Saturday.
Anti-government protests were suppressed in at least three other towns, the opposition said. In Butembo, in the eastern province of North Kivu, police fired teargas and threw stones at protesters, according to Fabrice Kakubuzi, a local civil society coordinator.
Provincial authorities in Kinshasa refused to authorise the march in the capital, organised by a coalition of leading opposition groups, arguing they had already permitted another rally on Saturday and could not secure two of them.
The international community has spent billions of dollars and deployed a 21,000-strong U.N. peacekeeping mission in Congo in the wake of a 1998-2003 civil war in which millions of people died mostly from starvation and disease. However, stability has remained elusive in the giant nation at the heart of Africa and dozens of armed groups still prowl its mineral-rich east.
Vital Kamerhe, president of the Union for the Congolese Nation (UNC) opposition party, said that police had teargassed protesters near Kinshasa's central train station. Several protesters were injured or arrested by police, he said.
Opposition supporters had voiced anger after UNC General Secretary Jean-Bertrand Ewanga was sentenced to one year in prison on Thursday for insulting Kabila at a protest in August. Authorities accused Ewanga of inciting tribal tensions by demanding at the rally that Kabila return to Rwanda when his second term expires in 2016 -- repeating an opposition accusation that the president was born in Congo's tiny eastern neighbour.
"We know that they have imprisoned Ewanga but they haven't imprisoned the spirit of Ewanga," Kamerhe said, vowing more protests.
A crowd of about 100 demonstrators gathered later outside UNC headquarters, some holding anti-government placards. "Kabila 2016: Let's avoid a Congolese Spring," read one.
A statement from the opposition movement, the Social and Political Forces for the Unity of Action, said that protests took place in several other towns in addition to Kinshasa and Butembo, including Bukavu in South Kivu province and Masi-Manimba in the west.
"The Social and Political Forces invite the entire Congolese population to participate massively, as today, in marches that they will organise all across the republic in the near future," it said, calling for its detained supporters to be released.
The suppression of the protests is likely to inflame tensions between Kabila's government and the opposition.
Allies of the 43-year-old president have grown increasingly vocal in their calls for an amendment to the constitution to remove a limit on the presidential mandate to two five-year terms. Kabila took office in 2001 shortly after the assasination of his father, then-president Laurent Kabila, before winning election for the first time five years later.
He won reelection in 2011 in a poll that was criticised by the U.N. peacekeeping mission for irregularities. Kabila has not publicly ruled out another run despite pressure from the United States to step down in 2016. (Reporting by Daniel Flynn; editing by Ralph Boulton)