Authorities in Democratic Republic of Congo arrested a prominent opposition leader on Sunday, the United Nations said, two days before a planned general strike that will call for President Joseph Kabila to step down from office this year.

Kabila is required by the constitution to stand aside in December after 15 years in power. Critics accuse him of trying to delay a presidential vote slated for November in order to stay in office. Dozens died in protests over the issue in January last year.

Martin Fayulu, president of the Engagement for Citizenship and Development (ECIDE) party and one of the organisers of Tuesday’s strike, was detained at his party headquarters in the early afternoon, said Jose Maria Aranaz, director of the U.N.’s Joint Human Rights Office (UNJHRO) in Congo.

Opposition leaders have called for all Congolese people to stay at home on Tuesday. It is not clear how well observed the strike is likely to be.

“It’s the same pattern of intimidation contrary to freedom of peaceful assembly enshrined in the constitution,” said Aranaz.

Aranaz said it was unclear if Fayulu had been arrested by the police or military. Blaise Munizi, a parliamentarian from ECIDE, said that Fayulu was in the custody of Congo’s intelligence services.

The government could not be immediately reached for comment.

(Reporting By Aaron Ross; Editing by Edward McAllister and Ros Russell)