By Anthony Boadle and Caroline Stauffer
BRASILIA/SAO PAULO (Reuters) - Brazilian prosecutors formally charged the treasurer of the ruling Workers' Party and 26 others with corruption linked to state-run Petrobras (SA:PETR4) on Monday, in the latest blow to President Dilma Rousseff from the widening scandal.
Prosecutors have "ample proof" that Joao Vaccari, treasurer of Rousseff's ruling party, solicited donations from former Petrobras services chief Renato Duque and executives at engineering firms accused of funnelling money from the oil company, prosecutor Deltan Dallagnol said.
Vaccari was "well aware" the donations he was seeking comprised funds stemming from bribes, Dallagnol said, adding that much of the evidence was gleaned from plea bargain deals with executives who were indicted and jailed late last year.
Such plea bargain deals have restored 500 million reais (103 million pounds) of stolen money to public coffers to date, he added.
The Workers' Party says all the donations it received were legal and Rousseff has denied knowing about corruption at Petrobras though she was chairwoman of its board from 2003 to 2010 when much of the alleged graft occurred.
Still, the scandal's fallout has heaped political pressure on Brazil's president months after she was narrowly re-elected. Outrage among Brazilians contributed to huge street protests across the country, pressuring her administration and in several cases calling for her impeachment.
Duque, who headed the department responsible for the bulk of Petrobras investments, was also indicted after federal police arrested him at his house in Rio de Janeiro. He had been detained for three weeks late last year in the early stages of the Petrobras investigation known as Operation Car Wash.
The arrest warrant was issued after prosecutors discovered transfers of about 20 million euros ($21 million) between accounts he managed in Switzerland and Monaco, along with transfers to U.S. and Hong Kong banks. Prosecutors said the amounts were "not compatible" with his earnings at Petrobras, formally known as Petroleo Brasileiro SA.
Transfers occurred up until the second half of 2014, after the investigation had started, a revelation the judge overseeing the probe in the southern city of Curitiba called "scary."
Duque's was one of six arrests being carried out by Brazil's federal police. They served a total of 18 search and detention warrants in Rio de Janeiro and Sao Paulo on charges that included bidding fraud.
Duque and Vaccari also face charges of money laundering and forming a cartel. Neither was immediately available for comment. Vaccari's lawyer has previously denied wrongdoing.
The corruption probe, Brazil's largest ever, had already led to 40 indictments on racketeering, bribery and money laundering charges.
Those previously indicted include two former Petrobras managers and 23 executives from six of Brazil's leading engineering firms OAS (OAS.UL) [OAEP.UL], Camargo Correa [PMORRC.UL], UTC Engenharia, Galvao Engenharia [QGDI.UL], Mendes Junior
Prosecutors have asked the Supreme Court to investigate 34 sitting politicians, including the speakers of both houses of Congress, for allegedly receiving bribe money.