Three buses that were transporting Nasa supporters from Mombasa to Nairobi have been intercepted in Voi, Taita Taveta County.
The buses were among six that left Mombasa for Nairobi ferrying the supporters to attend the swearing-in of Nasa leaders Raila Odinga and Kalonzo Musyoka.
Voi OCPD Joshua Chesire said the vehicles were seized for "traffic offences".
They were stopped at Voi at around 3am, according to one of the supporters.
"In the buses there were 82 passengers who also had not adhere to traffic rules. Most of them are youth of whom 10 did not have ID cards. Among them are four women," said Mr Chesire.
He added that the passengers would be taken to court and charged with traffic offences.
The buses were released at 8am, with Mr Chesire saying they could proceed with their journey.
However, the bus drivers told the Nation that they were going back to Mombasa and drove off towards Mombasa.
The Nasa supporters left Mombasa on Monday aboard six Modern Coast buses that awaited them at Treasury Square near Mombasa Governor Hassan Joho's office.
Traffic police sources said the buses' number plates had earlier been circulated to them to ensure they do not proceed with the journey.
The buses were flagged off by Mombasa County Speaker Harub Khatri, who was accompanied by the county ODM Chairman Mohammed Hatimy.
Speaking to journalists, Mr Khatri said the supporters were expected in Nairobi on Tuesday morning to witness Mr Odinga's oath taking.
"We are determined in this struggle. We will have at least 50 buses transporting about 3,000 supporters. This is the beginning of the journey to Canaan. No one has been forced but we are doing [this] for the love our leader," said Mr Khatri.
He said other buses would leave from Coast Bus offices.
Mr Khatri said Nasa supporters voted for Mr Odinga and Mr Musyoka in the August 8 General Election and the two leaders won but their victory was stolen by Jubilee.
"We know that we won that election and that is why we want to swear in our President - the people's president. We voted for him and we must see him take the oath," he added.
Mr Hatimy said no county funds was spent on transporting supporters.
"We want to put it clear that this is a party affair. Our speaker had promised to take our supporters to Nairobi and he has just fulfilled his promise. We will camp at Uhuru Park and see our leader taking the oath," he said.
During Mr Odinga's visit to Mombasa last week, Mr Khatri pledged to offer buses to transport the supporters to Nairobi for the Nasa event.
Police have warned people against going to Uhuru Park and its environs, saying they were not informed on the planned event.
Additional reporting by Lucy Mkanyika.