Italy's PM Silvio Berlusconi is facing a vote of confidence which could spark early elections.
He has already comfortably won the first round in the Senate, but the second vote in the lower house is expected to be extremely close.
Mr Berlusconi has urged MPs not to jeopardise the country's stability by ousting him.
But his critics say he is too mired in personal scandal and corruption allegations to remain in office.
Mr Berlusconi, 74, is halfway through a five-year term but his position has been weakened by a series of scandals largely involving his relationships with women.
He has also lost the support of his closest political ally, Gianfranco Fini, along with dozens of his supporters, depriving him of his automatic majority in the lower house.
Italy's high youth unemployment, budget cuts and a crisis over refuse collection in Naples have added to his problems.
Large protests are taking place in Rome and in major cities around the country. Several blasts were heard as the lower house vote was taking place. It was not immediately clear what caused them.
The BBC's Duncan Kennedy in Rome said Mr Berlusconi appeared looking very serious in the Senate as the final speeches were made, although his victory was almost certain.
The result was announced slightly earlier than expected, with Mr Berlusconi winning 162 of the 309 votes. Eleven senators abstained.
But victory in the lower house is less certain and could come down to one or two votes.
The atmosphere in the chamber prior to the vote was heated, as MPs presented their final speeches before the vote.
Antonio di Pietro, a former anti-corruption judge and now leader of the Italy of Values party, accused Mr Berlusconi of attempting to buy the vote.
He added: "One thing is for sure you don't have the majority that allows you to govern anymore, you just don't have it anymore."
His comments prompted an angry Mr Berlusconi - who has denied the allegations - to briefly walk out of the chamber, accompanied by jeering from opposition members.
"Go to the Bahamas! This is what awaits you: giving yourself up to the judiciary or fleeing," said Mr Di Pietro.
The MPs are now voting in alphabetical order in front of the rest of the house. Among the first to vote was opposition deputy Federica Mogherini, who is due to give birth imminently.
Another female MP, Giulia Bongiorno - loyal to Mr Fini's party - was pictured attending the vote at the lower house in a wheelchair due to complications linked to her pregnancy.
There were eruptions of cheers in the house after two of Mr Fini's supporters switched sides and voted in favour of Mr Berlusconi. There were also scuffled between other opposition MPs.
Correspondents say the vote rests with a few key MPs, but that even if Mr Berlusconi does win and manages to convince MPs to broaden the base of his coalition, it will be difficult for him to continue to govern with only a one or two-vote majority in the Chamber of Deputies.
'Crisis in the dark'
Mr Fini says Mr Berlusconi's personal scandals, gaffes and failed policy programme make his position untenable.
He says he has enough votes to unseat Mr Berlusconi, but the numbers are so close it is not clear which way parliament will go.
In speeches on Monday, the billionaire media tycoon said those seeking to remove him would be unable to form a government, and warned that early elections could cause political instability which would lead to a Greek or Irish-style economic crisis.
"I understand those who would challenge the government, opening a crisis leading to fresh elections or at least proposing a different prime minister, while being sure of the ability to form a new governing majority," he said.
"However, I cannot understand the spirit of those who want to trigger, at all costs, a crisis in the dark."
He said that under his government, Italy had gained a good reputation despite the financial crisis.
"I can say with absolute certainty that Italy is not part of the economic problems in Europe - it has become part of the solution," said Mr Berlusconi.
He has proposed a "legislative pact" with centre-right forces - members of Mr Fini's Future and Freedom for Italy (FLI) political movement, whose defections triggered the crisis, and the opposition Union of the Centre (UDC) party.
"I want to reconstitute the alliance of all the moderate forces that were the origin of our political engagement," he said.
An investigation has been launched into claims that inducements have been offered to some members of parliament to secure their vote for Mr Berlusconi.
If he loses the vote, Mr Berlusconi will have to tender his resignation to President Giorgio Napolitano.