KINSHASA (Reuters) - Police in eastern Democratic Republic of Congo arrested at least 17 democracy activists this week for campaigning against the postponement of the presidential election, fellow activists said, though 11 had been freed by Wednesday evening.

President Joseph Kabila has ruled Congo since 2001 and is required by the constitution to step down in December. But the ruling coalition and part of the opposition agreed this month to delay the vote to pick his successor from this November to April 2018, citing problems enrolling millions of voters.

Eight members of youth activist group Struggle for Change (Lucha) were arrested in the eastern city of Goma on Wednesday as they prepared to hold a sit-in near the headquarters of the U.N. peacekeeping mission, Lucha said in a statement.

Six other Lucha members were arrested in Goma on Monday for distributing tracts on a university campus calling for peaceful protests and three more who went to check on them on Tuesday were detained as well, the statement added.

A police spokesman confirmed the arrests, which he said were either for troubling public order or spying on police. He said the public prosecutor would bring charges against some of them on Thursday.

Georges Kapiamba, a lawyer for the Lucha members, said later that the 11 arrested on Tuesday and Wednesday had been freed.

Still, the United Nations said it was alarmed by the new crackdown. "We are very worried about the hardening of repression against civil society activities," Jose Maria Aranaz, director of the U.N. Joint Human Rights Office in Congo, told Reuters. "They were detained while raising awareness."

The government denies the arrests are politically motivated.

It released more than a dozen activists, including several Lucha members, in July and August in an attempt to appease the opposition and ease negotiations over the election timetable.

But the main opposition bloc boycotted the talks and violent demonstrations in the capital Kinshasa last month killed dozens of people. A U.N. report last week said security forces killed at least 48 civilians.

In its own report, the government denied that security forces were responsible for the deaths, which it blamed on the protesters themselves, private security guards and accidents.

(Reporting by Aaron Ross; Editing by Tim Cocks/Mark Heinrich)