BANJUL, March 22 (Reuters) - Gambian authorities have arrested several senior army officers and others have fled after being accused of a plot to overthrow President Yahya Jammeh, military sources in the tiny African country said on Wednesday. Jammeh cut short a trip to nearby Mauritania and returned home late on Tuesday, although no official reason was given for his early return and state radio did not mention the arrests. Soldiers armed with assault rifles stepped up security checks late on Tuesday on the Denton Bridge, which links the capital Banjul to the mainland, searching cars entering the city and causing long tailbacks on Wednesday morning. Elsewhere the town was calm and markets opened as usual. Military sources said a number of officers were arrested late on Tuesday night on suspicion of plotting a coup against Jammeh. There were no reports of shooting or violence, although the sources said a number of other officers had fled. As a young army lieutenant, Jammeh seized power in a bloodless coup in 1994 and is expected to seek a third elected term in elections due in October 2006. In February, the former wrestler dropped a case against three opposition leaders charged with acts including criticising the president, under a deal brokered by Nigerian President Olusegun Obasanjo to ensure peaceful elections. Opponents accuse Jammeh of a range of human rights abuses as well as rigging previous elections. He opened his campaign for the 2001 presidential polls by saying he had already won and threatening to shoot anyone who disrupted polling. Gambia, a former British colony of just 1.5 million people protruding into Senegal along the Gambia river, is very poor, depending mainly on groundnut farming, fishing and tourism.