NAIROBI (Reuters) - Kenyan police fired teargas and shots in the air on Monday as hundreds of demonstrators marched through the capital Nairobi to protest against proposed legal changes that would make it harder for the Supreme Court to annul an election. Reuters television footage showed a sport utility vehicle ploughing into some of the protesters, severely injuring three, but it was not immediately clear who was responsible and the police made no immediate comment on the incident.  Kenya is due on Oct. 26 to repeat a presidential election after the Supreme Court nullified an Aug. 8 vote due to procedural irregularities. President Uhuru Kenyatta, who won the August election, will face opposition leader Raila Odinga. Uncertainty over the coming election has created turmoil in the East African nation, which is a regional trade hub and staunch Western ally.  As the demonstrators marched toward the election board in Nairobi, a Reuters journalist saw men in plain clothes near security forces fire shots in the air. Police on horseback set up blockades to prevent protesters from accessing some roads.  Police also used teargas in the western city of Kisumu, Odinga s stronghold, to disperse protesters, though another demonstration in the coastal city of Mombasa passed peacefully. Odinga s opposition alliance is threatening to boycott the October vote unless the electoral board changes some personnel - a stance he declined on Monday to clarify, in comments that suggested he was keeping his options open for now.  This is a democratic society we live in. If I choose not to participate in the pre-rigged election process, it is my democratic right. No court can order me to do so,  Odinga said in comments broadcast on Kenyan TV stations, without elaborating further. The protesters want to warn ruling party lawmakers not to pass an amendment to the election law that would limit the circumstances in which the Supreme Court could void an election on procedural grounds, opposition lawmaker James Orengo said.   If parliament passes the law tomorrow, it will be like declaring war on the Kenyan people,  he said.  Justin Muturi, speaker of the National Assembly, said the amendment could be debated when parliament reconvenes on Tuesday if lawmakers were keen to push it forward.  He said the key part of the proposed law stipulates that if a candidate boycotts an election whose date has already been set, the remaining candidate would win the contest without formal polls being held. The government is keen to avoid a constitutional crisis if Odinga pulls out at the last minute, frustrating the court s order to hold elections within 60 days, Muturi said.   We are trying to prevent a crisis should there be no elections,  he told Reuters by phone.  The government-backed Kenya National Commission on Human Rights said on Monday in a report that at least 37 people had been killed in a police crackdown on protests that immediately followed the August vote - the highest death toll given so far. The report attributed some deaths to  police using live bullets and a few from police bludgeoning using clubs . It named a 6-month-old baby girl, a 7-year-old boy, and an 8-year-old girl as being among the victims. 