Defeated Kenyan presidential contender Raila Odinga is challenging the result of the March 4 election.
Odinga, who is Kenya’s current prime minister, narrowly lost out to Uhuru Kenyatta – son of Kenya’s first president, Jomo Kenyatta.
Odinga has filed a petition with the country’s Supreme Court to veto the election, which he says was not free or fair.
Crowds of Odinga supporters gathered in Nairobi backing his call for the court to nullify the election result.
Police used to tear gas in an attempt to break up the demonstration.
The election has been largely peaceful – unlike the last presidential vote in 2007. The disputed result sparked two months of bloody clashes in which more than a 1,000 people were killed.
Uhuru Kenyatta and his running mate William Ruto are both facing charges brought by the International Criminal Court – accused with helping to orchestrate the post-election violence in 2007-8.