Police in the Thai capital blocked a march Tuesday by pro-democracy activists protesting four years of military rule and detained several of the movement's leaders. About 3,200 officers were deployed, police said, to prevent about 200 demonstrators from defying a junta ban on protests with a march from a Bangkok university campus to Government House, where they were to demand that elections be held this year. Faced with barricades, a tropical downpour and lines of police, the protesters dueled with authorities by loudspeaker, trying to outshout police warnings that the demonstration would tarnish Thailand's image, scare tourists and cause congestion. Activist Seriwith Seritiwat said the protesters would "never back down," and tempers briefly flared as they pushed and shoved with police. By late afternoon the demonstrators began to disperse after police said they were charging protest leaders with violating the law on assembly. Three of he leaders turned themselves in. Tuesday was the fourth anniversary of a bloodless coup in 2014 that toppled Thailand's elected government, the country's second coup in less than a decade. The junta vowed reform and reconciliation for a politically divided Thailand but its rule has been tarnished by corruption scandals and repeated postponement of promised elections. The protesters, mainly middle-aged and elderly people led by a core of student activists, have been holding regular rallies for the last few months, calling for the junta to resign. Its most recent promise of elections is for February next year, but the protesters want the junta to stick to an earlier pledge to hold them in November. Political gatherings of five or more people are banned by the military government, which has regularly summoned its opponents to military bases for what it calls "attitude adjustment." Amnesty International used the anniversary of the military takeover to reiterate its calls for the junta, known formally as the National Council for Peace and Order, to restore civil rights in the country. "The sweeping and wholly unjustified restrictions on human rights put in place by the NCPO in the wake of the coup were supposed to be exceptional and temporary measures," Katherine Gerson, the group's Southeast Asia campaigner, said in a statement. "Four years on and countless abuses later, they remain firmly in place and are relentlessly deployed by authorities." Analysts say junta leader Prayuth Chan-ocha, who led the coup and is now prime minister, is keeping a tight grip on political activity and dissent and is maneuvering to retain power even if elections are held.