Harare - Hundreds rallied in Zimbabwe's capital on Wednesday over the shadowy disappearance of an opposition activist a year ago, as the United States led calls for a probe into "politically motivated violence".Zimbabwe's 92-year-old ruler Robert Mugabe, who has led the country since independence from Britain in 1980, has been accused by critics at home and abroad of brutally cracking down on opponents and smothering democracy.Protesters including opposition leader Morgan Tsvangirai demanded that Mugabe release information on how Itai Dzamara, a harsh regime critic, was seized by unidentified men.Dzamara was the leader of an anti-government campaign group that sought to force Mugabe to resign over the collapse of the economy, largely sparked by the seizure of white-owned farms which led to a dramatic fall in agricultural production.On March 9 last year, Dzamara was bundled into an unmarked car while coming out of a barbers shop and he has not been seen since."Why should the regime resort to violence whenever the people want to express themselves?" Tsvangirai told the rally in Harare's African Unity square, where Dzamara had staged sit-in protests and was once beaten by pro-Mugabe supporters."We will hound this government forever and ever until they bring Itai to us alive or dead.""Stop abductions now", read a placard at the protest while another demanded that authorities "End forced disappearances now".Several foreign countries have repeatedly pushed the government to reveal the fate of Dzamara.Troubling indictment "The United States remains deeply concerned about Mr Dzamara's whereabouts and wellbeing," the US embassy said in a statement."The Zimbabwean constitution guarantees fundamental human rights and freedoms for all citizens, including Mr Dzamara."We also encourage the government of Zimbabwe to fully investigate cases of politically motivated violence and abductions to ensure that perpetrators are prosecuted and victims receive justice."The European Union expressed disappointment at that Dzamara's disappearance had so far remained unsolved."Those responsible for his abduction have yet to face justice," it said."The EU attaches great importance to strengthening the rule of law, human rights, and the consolidation of democracy in Zimbabwe, as set out in the government's own constitutional reform agenda."The government has denied accusations that it was involved and said it has no information on Dzamara's whereabouts.Amnesty International called on the authorities to "genuinely look" for Dzamara."This appears to be a well-orchestrated plot to silence a well-known government critic, and is a deeply troubling indictment of the state of freedom of expression in Zimbabwe," said Deprose Muchena, the group's director for southern Africa.