Brasilia (Brazil) (AFP) - Brazilian police launched a new round of arrests Monday in the corruption scandal at state oil giant Petrobras, a day after massive nationwide demonstrations against leftist President Dilma Rousseff.

Federal police said they had warrants for the arrest of 18 people in connection with the 10-year scheme of kickbacks and political payoffs that allegedly siphoned off $3.8 billion from Petrobras.

"The warrants are being carried out in the cities of Sao Paulo and Rio de Janeiro," a police statement said.

Among those targeted for arrest was Petrobras's former director of services, Renato Duque, allegedly a key figure in the scheme.

On Sunday, an estimated 1.5 million people took to the streets across Brazil in protest over the corruption, with many calling for Rousseff's impeachment.

Rousseff, who was re-elected to a second term in December but whose poll ratings have since plunged, chaired the Petrobas board during much of alleged corruption period.

She is not being investigated, but leaders of her ruling coalition in Congress have been named in the probe, along with a who's who of Brazilian politicians.

The list of 49 politicians under investigation is headed by Senate president Renan Calheiros and Speaker of the Chamber of Deputies Eduardo Cunha, both leaders of the centrist Brazilian Democratic Movement Party, or PMDB, a key component of Rousseff's ruling coalition.

So far, 13 senators, 22 deputies and two governors have been placed under investigation.

Those arrested in Monday's operation were private citizens caught up in the scandal, not public servants.

Many of Brazil's largest construction companies have fallen under suspicion as investigators unravel a web of inflated contracts that generated a river of dirty money allegedly used to bribe Petrobras executives and pay off politicians.