Kenyan police fired in the air on Friday to disperse hundreds of opposition supporters gathered outside the airport in the capital, Nairobi, a Reuters witness said.
Despite the police's partial ban on protests in the capital, the demonstrators gathered near the airport to wait for the return of their leader, Raila Odinga, from an overseas trip.
The demonstrators threw rocks at police trying to disperse them, the witness said.
Police were checking vehicles headed from the city center to the airport in another attempt to stop the demonstration from building.
Odinga has called for a "National Resistance Movement" to protest the outcome of the recent repeat election, which was ordered by the Supreme Court after it annulled the results of an August poll over procedural irregularities.
President Uhuru Kenyatta won a second, five-year term with 98 percent of the vote after Odinga boycotted the contest. Only 39 percent of registered voters took part.
The court will rule on Monday on cases that seek to nullify last month's repeat vote.
Judges could order a fresh vote or clear the way for the incumbent to be sworn in for a second term.