Several ODM activists were detained before President Uhuru Kenyatta's rally at Voi.
Police and local politicians said the activists were detained at local police stations for the entire period the President was in the county. They were freed without charge after the function.
At the rally, Senator Dan Mwazo and Woman Representative Joyce Lay were welcomed to the Jubilee Party from ODM.
The rally held at Moi Stadium in Voi also saw residents heckling the defectors, with some describing Ms Lay as a traitor.
Tuesday, ODM county leaders led by Governor John Mruttu, accused the police of breaching the Constitution and harassing and intimidating the party's local leaders.
Police admitted that the activists were detained after intelligence reports indicated that the opposition was plotting to disrupt the President's function and were freed without charge.
County Police Commander Fred Ochieng said only two people were arrested and interrogated for allegedly intending to disrupt the presidential rally.
"We received a tip-off that some people were planning to disrupt the presidential rally. We summoned them to verify the validity of the information because the claims were a security threat," said the police commander.
He added: "We summoned the suspects for interrogation to preempt their intentions. We questioned them in good faith and later released them without preferring any charges against them."
Mr Mruttu, however, said five activists were arrested and named them as including former civic leader Eric Ochieng, Voi sub-county boda boda riders chairman Mohammed Dudu, and ODM activist Alphan Mborio Mwachofi.
He claimed that Assembly Deputy Majority Leader John Maghanga and Kaloleni ward aspirant Richard Babu were also detained, but the police commander denied knowledge of their arrest, insisting that only Mr Dudu and Mr Mwachofi were detained.
The opposition leaders accused the police of detaining the leaders on the orders of a local Jubilee leader.
"We do not get commands or directives from politicians to discharge our mandate. We only get directives from our seniors," Ochieng said, adding that security personnel cannot ignore any intelligence touching on security.
"The questioning of the leaders was not in bad faith. It was an act of gathering intelligence," he added.
"We were summoned by the DCI department and interrogated for hours. We were told that we were a threat to security and I spent the night at the police station," said Dudu adding: "I will not be cowed and intimidated for supporting Raila Odinga."