Romania's chief anti-corruption prosecutor says a bribe probe against the influential mayor of Bucharest began months ago.

Laura Codruta Kovesi said Tuesday prosecutors stopped mayor Sorin Oprescu late Saturday in traffic and detained him on bribe allegations which first emerged in March.

Oprescu was later arrested on suspicion that he took bribes of 25,000 euros ($28,000) from firms working for city hall. He is the most senior politician to be arrested on corruption charges this year.

The anti-corruption prosecutors' office said that the firms paid bribes to high-ranking city hall officials from their profits. About 10 percent allegedly went directly to Oprescu.

Oprescu denies he ever solicited bribes. Four others have been arrested in the same case.

The 63-year-old Oprescu has been Bucharest mayor since 2008.