Human rights lawyer Harun Ndubi was arrested on Sunday night, police alleging drink-driving.
Ndubi was apprehended near Gate D of State House after his car stalled on Jakaya Kikwete Road. He had been on his way home when he was found asleep in the vehicle.
Officers said they tried to force him to take a breathalyser test, to check his alcohol concentration, but that he declined.
He spent the night at Kileleshwa police station and will be arraigned on Monday morning.
But the lawyer’s family and other human rights activists reported that he was drugged by unknown people.
Ndubi has been in the forefront of the push for electoral reforms by the opposition and civil society activists.
He is currently representing Dagoretti North MP Simba Arati whose firearm licence the government revoked, amid a crackdown on National Resistance Movement members.
Arati is a member of the movement led by National Super Alliance boss Raila Odinga.