* Kabila term expires Monday, no new election organised
* Talks with opposition stall, fears of violence
* Government blocks social media, sets up checkpoints (Adds Kabila chief diplomat, more arrests in Goma, Kinshasa, Cape Town protests)
By Aaron Ross and Tim Cocks
KINSHASA, Dec 19 (Reuters) - Protesters gathered in Democratic Republic of Congo's capital and at least one other city on Monday amid a heavy military presence before protests due once President Joseph Kabila's mandate runs out at midnight.
Hundreds of anti-Kabila demonstrators defied a ban on marches against the president's plans to stay in office past the end of his term. Security forces blocked access to Kinshasa University, facing off against groups waving red cards saying "Bye, bye Kabila," as time ticked down to midnight.
Militia fighters raided a jail in eastern Congo's Butembo trying to free prisoners, triggering clashes that killed a South African U.N. peacekeeper and a police officer.
Seven attackers were also killed, Kabila's chief diplomat Barnabe Kikaya told a news conference in Kinshasa.
Opposition activists have accused Kabila of trying to cling to power by letting his term run out without an election to chose the next leader of Congo, which has not witnessed a peaceful change of power since independence in 1960.
"Kabila's mandate finishes at 1159 ... Tomorrow (Tuesday) it will be chaos," said Hugue Ilunga, 21, as dozens of soldiers deployed nearby in the capital, an opposition stronghold of 12 million people.
Shops shut in other parts of Kinshasa, where streets were largely empty.
At least 80 protesters were arrested in the eastern city of Goma, the U.N. human rights office in Congo said, mostly activists who were simply wearing red shirts, an opposition colour. Police said nine opposition demonstrators were detained.
Kabila's elite Republican Guard also arrested prominent opposition activist Franc Diongo in Kinshasa, Kikaya said, after Diongo's private guards beat up three of them.
The government and elections officials have blamed logistical and financial problems for the delay in the vote, currently scheduled for April 2018.
Some opposition leaders agreed Kabila can remain in office until then. The constitutional court has also ruled that Kabila, leader since his father was assassinated in 2001, can stay on.
But the main opposition bloc rejects the deal as a ploy, though it said it would not call protests. Talks mediated by the Roman Catholic church failed to reach a compromise.
"Joseph Kabila will remain in office tomorrow," Kikaya said.
Authorities have blocked most social media and outlawed protests in Kinshasa, raising fears of more violence in a nation that has been plagued by war and instability for two decades since the fall of kleptocrat Mobutu Sese Seko.
Diplomats fear any escalation could trigger a conflict like the 1996 to 2003 wars that killed millions, sucked in neighbouring armies and saw armed groups clash over Congo's mineral wealth and use of mass rape as a strategic weapon.
U.S. Great Lakes envoy Tom Perriello on Thursday called Kabila's hanging on "an entirely unnecessary flirtation with disaster" in a speech at the United States Institute of Peace.
Youth activists say they have taken inspiration from Burkina Faso in West Africa, where protests ousted Blaise Compaore in 2014 as he was trying to extend his 27-year rule.
As in Burkina, protests in Congo are in part driven by economic desperation. Congo is Africa's biggest miner of copper and metals used in gadgets, like cobalt and coltan, but a slowdown linked to falls in commodity prices has triggered steep budget cuts and a 30-percent fall in the Congolese franc.
The country of 70 million people and more than 200 ethnic groups is fragmented, though. Previous protests achieved little.
Former colonial master Belgium advised its citizens to leave before Monday. The United States warned against non-essential travel, telling expatriates who remain to stay indoors.
Outside Congo, South African police used stun grenades to disperse anti-Kabila protesters in Cape Town. (Additional reporting by Ed Cropley in Johannesburg and Amedee Mwarabu in Kinshasa; Editing by Tom Heneghan)