Kinshasa - More than 2 000 people have marched down a main boulevard in the Democratic Republic of Congo's capital, calling on Belgian authorities to release warlord-turned-political leader Jean-Pierre Bemba.

"Free Bemba! Free Bemba!" the crowd shouted as it pushed towards the parliament building on Tuesday, blowing whistles and brandishing posters of the man who was the runner-up in the nation's 2006 presidential election.

Belgian police arrested Bemba on Saturday on war crimes charges stemming from his time heading a militia that allegedly committed atrocities in the DRC's 2002-2003 conflict.

The International Criminal Court in The Hague had issued the arrest warrant on charges of rape and torture that were kept secret until he was captured.

Bemba, who is in his mid-40s, vowed to remain in the government as an opposition leader after losing the presidency to Joseph Kabila, and was elected as a senator.

However, he was forced to flee to Portugal in April 2007 after being accused of treason in the DRC. He also acquired a home in Brussels.

Officials of Bemba's political party said the arrest was unjust and unnecessary.

"Bemba isn't a hoodlum," said Francois Muamba, the secretary-general of Bemba's party. "If they want to talk to him, they can present themselves wherever they want."

Party leaders said they had formally asked parliament to condemn the arrest and urge a foreign government to respect the immunity that the DRC grants to senators.

The court's prosecutor "wants to interfere with the internal businesses of Congo and that is unacceptable", said Delly Sesanga, an official with Bemba's Movement for the Liberation of Congo party. Sesanga said Bemba had not given any indication that he was a flight risk, as prosecutors alleged.

The crowd of protesters in Kinshasa continued to grow at midday. Men and women on foot were followed by cars filled with Bemba supporters.

Police were stationed outside the Belgian embassy to prevent protesters from charging the building. - Sapa-AP