Tanzania is holding about 500 suspected sex workers and nearly 300 of their alleged customers in detention as part of a crackdown on prostitution.
The first suspects will start appearing in court on Wednesday, Dar es Salaam's deputy police commissioner told the BBC, confirming the arrests.
Rights groups have condemned the arrests, which started a week ago, urging police not to hold suspects beyond the legal 48-hour limit.
Prostitution is illegal in Tanzania.
Tanzania's new President John Magufuli, who came to power on an anti-corruption platform, has previously criticised prostitution in the country.
President Magufuli, known as the Bulldozer, has vowed to uphold law and order since taking power in November.
Deputy police commissioner Simon Siro told BBC Swahili that the suspects were being held in three prisons around the city.
He said the sex trade was linked to both drugs and violent crime in Tanzania.