N'DJAMENA, May 8 (Reuters) - Chadian security forces fired tear gas to disperse protesters who took to the streets of the capital N'Djamena on Saturday to protest against a military takeover that followed the death of President Idriss Deby on the battlefield last month. On Friday the transitional military government, led by Deby's son Mahamat Idriss Itno, had banned the protest called by a coalition of some opposition parties and civil society groups. A strong ally of Western nations in the fight against Islamist militants in western and central Africa, Chad has seen heightened political tensions since Deby's death. Although the military has promised to organise elections within 18 months, some opposition parties have rejected the military-led transitional government, calling it a coup and a continuation of Deby's 30-year rule.