Amnesty International on Tuesday called for the "immediate and unconditional" release of Maurice Kamto, leader of Cameroon's opposition MRC, after he was arrested in the country's economic capital Douala.Kamto, who came second in October's presidential election which he claims was rigged, was detained on Monday at the home of another politician, prompting some 300 people to protest outside.Party treasurer Alain Fogue was also arrested on Monday and over the weekend, Paul-Eric Kingue, who ran Kamto's election campaign, and rapper Valsero were also detained alongside 115 others for attending opposition protest marches in several towns.Amnesty International warned that the arrest of Kamto and his staff "signals an escalating crackdown on opposition leaders, human rights defenders and activists in Cameroon"."The authorities must immediately and unconditionally release them, as well as peaceful protesters detained at the weekend simply for exercising their rights to freedom of expression and peaceful assembly," said Amnesty's West and Central Africa director Samira Daoud. "Instead of taking steps towards improving the country's human rights record, we are witnessing the authorities becoming less and less tolerant of criticism. This must stop."Kamto's Movement for the Rebirth of Cameroon, denounced his detention as an "unjustified" political arrest and accused the government of trying to "decapitate" the party."(The party) strongly condemns these unjustified... political arrests whose hidden aim is to decapitate the MRC and Maurice Kamto's winning coalition," party vice president Emmanuel Simh said in a statement.REDHAC, an NGO representing human rights defenders in Central Africa, also denounced Kamto's arrest as a "flagrant violation" of the law, demanding his "immediate and unconditional" release along with the others detained at the weekend.President Paul Biya, who has ruled Cameroon for 36 years, won a seventh consecutive term in October's presidential election but Kamto has insisted he was the rightful winner.Official results gave 85-year-old Biya 71% of the vote, compared with 14 percent for Kamto, and the party has since staged a series of sporadic protests. 0Communications Minister Rene Emmanuel Sadi accusing Kamto and the MRC of trying to "destabilise" the government, and following Saturday's demonstrations, Paul Atanga Nji, minister for territorial administration, said the party had "crossed the red line".Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.