KINSHASA (Reuters) - A Congolese opposition leader was arrested late on Sunday for his role in anti-government demonstrations last month in which more than 50 people died, the government said on Monday, leading to threats of more protests if he is not released.

Bruno Tshibala, the deputy secretary-general and spokesman for the Union for Democracy and Social Progress (UDPS), Democratic Republic of Congo's largest opposition party, was arrested Sunday night at Kinshasa airport as he prepared to board a flight for Brussels, the UDPS said in a statement.

Government spokesman Lambert Mende confirmed the arrest, saying Tshibala would be questioned by the attorney general's office over his role in the protests, including the deaths of a police officer who was burned alive and a young girl.

"He is among the people who organized the protests of (September) 19th and 20th," Mende told Reuters. "Tshibala signed a communique in which he congratulated the protesters."

Congo's attorney general has said he will punish those behind the demonstrations, which were protests against President Joseph Kabila's alleged attempts to retain power beyond the end of his final term in December.

The secretary general of the UDPS, Jean Marc Kabund, promised protests on Wednesday if Tshibala was not freed.

"If he is not liberated tomorrow, you can count on UDPS being in the streets the following day," he told Reuters.

Kabila, in office since 2001, is barred by constitutional term limits from standing for a third elected term. But the country's electoral commission says logistical and budgetary obstacles will prevent it from organizing a new presidential election before December 2018.

(Reporting By Aaron Ross; Editing by Larry King)