The leader of Brazil's main opposition party says embattled President Dilma Rousseff is losing her grip on power amid a corruption scandal that is compromising her ability to pull Latin America's biggest country out of its worst recession in decades.

Senator Aecio Neves, the leader of the Social Democracy Party who narrowly lost to Rousseff in a 2014 presidential runoff, says the Brazilian government "has lost its legitimacy, the ability to run the country."

Impeachment proceedings over allegations that Rousseff's administration infringed fiscal rules have eroded her political support and made it hard for her to pass legislation that could improve the economy.

Neves is attending a legal conference in Lisbon, Portugal, on Thursday that included other participants seen as opponents of Rousseff.