Kenyan security forces cracked down on demonstrators on Saturday, as crowds gathered to support the defeated presidential candidate Raila Odinga's supreme court challenge to the results of the 4 March elections.

Odinga refuses to concede to Uhuru Kenyatta, whom Kenya's Independent Electoral and Boundaries Commission declared winner with 50.07% of the vote. Odinga's lawyers have filed a petition alleging the elections were not free and fair due to irregularities in the commission's methods.

Odinga accused the commission of "criminal neglect" during the election before sending his lawyers to court. Odinga said the commission was at fault for lack of transparency, discrepancies in vote tallies and registers, and the breakdown of electronic voter identification kits and results transmission systems. "At many polling stations, the valid votes cast exceeded the number of registered voters," he further alleged. "These failures dwarf an thing Kenyans have ever witnessed in any previous elections."

Hundreds of Odinga supporters in white shirts bearing the slogan Democracy on Trial gathered by the court in Nairobi to support the petition. The situation grew tense as Kenyan security forces turned up on horseback and in riot gear. Just before 10.30am, they fired teargas at the protesters and shot bullets into the air. The Observer saw one demonstrator and a bystander bloodied by security forces armed with batons and rifles.

Josiah Augo, 25, a student at Kenyatta University who joined the demonstration, said: "We were here peacefully and the police come with teargas. People started running, then one officer shot a student."

Odinga's running mate, Kalonzo Musyoka, appealed for calm. "I want to call on the police to exercise restraint," he said. Kenya's police inspector general said through a spokesman that further public political demonstrations and prayer gatherings would be banned. After Odinga's lawyers filed their petition, demonstrators returned to the streets, taunting security officers with blood-stained T-shirts. Odinga lawyer Dalmas Otieno addressed the crowd. "We have got enough information to show that our victory was stolen," he shouted to cheers.

Kenya has stayed peaceful during the elections and their aftermath, in contrast to general elections in 2007 when over 1,200 people were killed. But frustration has built among some Odinga supporters over commission missteps. Last night students at Kenyatta University rioted and burned a vehicle after finding a room full of commission documents including ballot boxes and unused ballots.

The commission said the materials were left behind from a mock election held a month ago.

Odinga and Kenyatta both said on Saturday that they would accept the court's decision, which will be delivered within two weeks.

Odinga has called for a rerun of the election, but if the court finds Kenyatta's lead slips below 50%, there will be a runoff. Kenyans widely respect their independent judiciary, which was strengthened by the country's new constitution, and Odinga urged his supporters be peaceful. "Violence disrupts the lives of the innocent and deeply divides," he said.