Kinshasa police said on Monday that trouble-making infiltrators had triggered fresh tension at the city university, three days before the kickoff of campaigning for next month's presidential election."Infiltrations of thugs from Mbanza-Lemba are behind the troubles at the University of Kinshasa," police chief Sylvano Kasongo told AFP in reference to a poor neighbourhood located near the school."Police have restored law and order," he added.Detained students have now been released, but "the thugs remain in detention and will be brought to justice," Kasongo said.A biology student told AFP that police fired live ammunition into the air and tear gas in areas where the students lived."We threw stones to defend ourselves," the student said.Another science student who said he had fled back to his room claimed that "police officers are all over campus. They arrest passers-by, and rob their telephones, money and bags." One Western embassy warned compatriots by text message to avoid the area "at least for the day" because "stone-throwing was taking place".Campaigning for the country's December 23 election is to kick off on Thursday.Last week, a police officer shot dead two students during campus protests, and Kasongo has pledged that he would be "tried and sentenced".University students have been demonstrating for the past week against a strike by professors over pay issues, now in its third month.Seven opposition leaders have picked little-known MP Martin Fayulu as their joint candidate in the December 23 vote to succeed President Joseph Kabila, in power for 18 years.The election is critical for the future of the DRC, a sprawling, mineral-rich country that has never known a peaceful transition of power since independence from Belgium in 1960.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.