Two people were killed on Friday as Kenyan police tried to disperse supporters cheering a convoy carrying opposition leader Raila Odinga from the airport to central Nairobi, a Reuters photographer said.
Police fired tear gas at the convoy and used water cannon as they tried to stop it reaching the capital's main business district, live footage on Kenyan TV showed. Some protesters threw stones at police. TV footage showed two vehicles on fire.
The deaths occurred as police and protesters fought on one of the main roads leading to the business district, the photographer said. Police spokesmen did not immediately respond to phone calls.
Odinga has called for a "National Resistance Movement" to protest against the outcome of a repeat presidential election last month which saw President Uhuru Kenyatta win 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 repeat poll was ordered by the Supreme Court after it annulled the results of the August election, won by Kenyatta, over procedural irregularities.
In what is seen as the last chance for legal scrutiny of the vote, the court will rule on Monday on cases that seek to nullify the rerun election.
The political crisis has stirred fears for the stability of the east African nation, a regional hub for trade, diplomacy and security.
Earlier on Friday, demonstrators threw up burning barricades on Mombasa Road, the highway that links Nairobi's downtown business district and the airport.
Odinga returned to Kenya on Friday from a trip to the United States.
Despite a partial police ban on protests in the capital, hundreds of people had gathered in the morning near Jomo Kenyatta International Airport to await Odinga's return. Police fired into the air to disperse them, a Reuters photographer said.
Once Odinga landed, motorcycle taxi drivers and people on foot shouted their support as the motorcade, which included lawmakers from Odinga's opposition coalition, began moving toward central Nairobi.
In Monday's Supreme Court rulings, judges could order a fresh vote or clear the way for the incumbent to be sworn in for his second term. If the election is upheld, Kenyatta will be sworn in on Nov. 28.
Dozens of international flights depart and arrive daily at Nairobi's main airport. The national airport authority said on Twitter just before midday that operations were running normally.
Kenya's prolonged election season has disrupted its economy. Human rights groups say at least 66 people have died in bloodshed surrounding the two elections.
Ahead of Monday's Supreme Court ruling, Kenya Airways' chairman told an investor briefing on Friday: "Hopefully we don’t have another presidential election so we can get on with life."