More than a million people are expected to flood Brazil's streets Sunday in massive anti-government protests calling for President Dilma Rousseff's ouster over a corruption scandal and the crumbling economy.

Latin America's biggest country, which will host the Summer Olympics in Rio de Janeiro this August, is in severe economic and political crisis, with fears that the demonstrations could trigger clashes.

‘I am appealing for there not to be violence,’ Rousseff told Brazilian media late Saturday. ‘I think all people have a right to be on the streets. However, no one has a right to be violent. No one.’

The largest demonstration is expected in Sao Paulo, the country's financial capital and main opposition stronghold. Authorities said they were bracing for a million protesters.

Earlier in the day, large demonstrations are to kick off in Rio and Brasilia. Organizers said some 400 cities in all would stage protests.

‘This popular will is organized and gaining influence,’ said Bonifacio de Andrada, a deputy from the opposition PSDB party. ‘People will take to the streets for large demonstrations to demand a change of government.’

Rousseff -- deeply unpopular because of a giant corruption scandal centered on state oil company Petrobras and because of her management of the country's worst recession in decades -- faces impeachment in Congress.

Her chief mentor in the leftist Workers' Party, ex-president Luiz Inacio Lula da Silva, is fighting allegations that he was part of the Petrobras corruption network. As if that weren't enough, the PMDB party, Rousseff's crucial partner in a shaky government coalition, indicated Saturday that it could pull out in 30 days.

- To the streets -

Criminal charges filed against Lula on Wednesday for allegedly undeclared ownership of a luxury apartment have invigorated the opposition and increased chances for an impressive turnout. A single call on Facebook for Jiu-jitsu and other martial arts fans to attend the Rio march was viewed more than 214,000 times by Saturday.

In Sao Paulo, yoga instructor Ruben Caetano, 45, told AFP he was attending in ‘hope of changing the way things are in the country. We are living in a time of a lot of corruption. Corruption permeates all levels of public office in the country. So it's not against party A, B or C.’

For deputies in Congress of all stripes, crowd size will be a crucial signal for whether or not to push for Rousseff's impeachment.

Opposition movements like Go to the Streets are so well organized that anything less than very large turnout would likely be seen as failure. The biggest anti-government protest last year, in March, saw an estimated 1.7 million people across Brazil, with a million in Sao Paulo alone. Another six months later some 1.2 million people attended.

Supporters of Rousseff have backed away from a threatened counter demonstration in the capital Brasilia, but with tensions in this divided country growing by the week and compromise seemingly off the table, there were fears of trouble.

Rousseff, a former leftist guerrilla tortured under Brazil's dictatorship, insists there isn't ‘the slightest possibility’ she will resign.

And Lula, who founded the Workers' Party and became one of the world's most popular politicians during his 2003-2010 presidency, is fighting not just for his political future but freedom. Sao Paulo state prosecutors this week asked a judge to authorize preventative detention for the powerful figure.

‘If they want to defeat me, they will have to face me in the streets,’ Lula said.

- Power struggle -

Congress is mulling impeachment on the grounds that Rousseff allegedly manipulated government accounts so that she could illegally boost public spending during her 2014 re-election campaign. The Supreme Court is due to rule on technical issues that have temporarily held up the impeachment proceedings.

As pressure builds, Rousseff is running out of allies.

Not only is the PMDB threatening to abandon her, but its leader -- her vice president, Michel Temer -- would become interim president were she to be impeached.

Against that backdrop, Rousseff needs help from Lula more than ever. Although now a highly divisive figure, Lula gives Rousseff credibility with swaths of voters who remember his success in bringing millions of Brazilians out of extreme poverty.

In the latest of a series of chess-like maneuvers with the opposition, Rousseff has said she is considering giving Lula a ministerial post.

This would put him out of reach of regular criminal courts, because sitting politicians can only be judged in the Supreme Court.

However, Lula is said to be reluctant, given that such a move would make him even more of a hate figure to the opposition, escalating the power struggle.