Athens (AFP) - Prime Minister Alexis Tsipras urged lawmakers in the early hours of Thursday to vote through reforms that must pass if Greece is to unlock a huge international bailout, saying the country must confront "new realities".

The vote poses a fresh challenge to Tsipras' authority, after he suffered a major rebellion by MPs in his leftist party Syriza last week during a first vote on tough reforms demanded by Athens' international creditors.

Tsipras had to rely on opposition MPs to pass the first bill, which approved sweeping changes to debt-crippled Greece's taxes, pensions and labour rules -- key conditions for opening talks on a bailout worth up to 86 billion euros ($93 billion) over three years.

In a passionate speech to parliament ahead of a vote on the second bailout bill early Thursday, Tsipras said he was determined to keep fighting for Greece's best interests, adding he would not "voluntarily abandon" the job.

"The left's presence in government is a bastion for the defence of the interests of the people," he said.

"We will not be cowards. We will fight the battles we face with determination."

Some 6,000 anti-austerity demonstrators gathered near parliament ahead of the vote to protest against the cash-for-reforms deal, according to police.

A handful of petrol bombs were lobbed in the direction of the security forces, who had thrown up a ring of steel around parliament after riots during last week's vote on austerity measures.

Tsipras was expected to win Thursday's vote comfortably after opposition parties said they would back it, but analysts said the debate would show whether the premier can avoid another deep split within his own party and head off the risk of early elections after only six months in power.

Katerina and George Sergidou Kokkinavis, two Syriza members in their thirties taking part in Wednesday's protest, said they had come because "the government is no longer listening to the people".

Syriza came to power on an anti-austerity ticket however the government has agreed to a deal that involves more painful reforms -- despite a referendum result in which Greeks came out against further cuts -- in the hope that it will prevent the country from crashing out of the eurozone.

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- 'The pain doesn't stop here' -

The second bill, less controversial than the first, covers civil justice reforms, a bank deposit protection scheme, and measures to shore up the liquidity of the banks, which reopened Monday after a three-week closure imposed to avert a catastrophic bank run that could have seen Greece's financial system collapse.

The banks have seen some 40 billion euros withdrawn since December by customers anxious over the safety of their cash -- seriously undermining the banks' ability to function normally.

Bloomberg News reported that the European Central Bank boosted its cash lifeline for Greek banks -- the programme known as Emergency Liquidity Assistance -- by 900 million euros on Wednesday, in a move that had been announced last week.

The ELA is the only source of funding for banks until the bailout package can take effect -- and, by extension, the sole financial lifeline preventing the Greek economy from imploding.

Tsipras told parliament that Greece would have to adapt to "the new realities" of the bailout agreement.

"The painful path does not stop here, unfortunately," he said. "We have been forced to make a difficult compromise under urgent conditions. We were at the limits of our economy and our banking system, and at the limits of Europe -- where conservative forces, obsessed with austerity, dominate," he added.

Tsipras was forced to reshuffle his cabinet last week, having sacked three ministers for voting against the first bailout bill.

Vassiliki Georgiadis, a political science professor at Athens' Panteion University, said the split within Syriza was between hard-left MPs -- "some of whom have spoken of a Greek exit from the eurozone as the only solution" -- and those more sympathetic to Tsipras's arguments.

She added that it would be "difficult" for the 40-year-old premier to continue in office if he was forced to rely on opposition MPs to get laws passed.

Government spokeswoman Olga Gerovassili said Monday that elections would not be "useful at the moment", and the government had "no intention" of calling early polls.

Gerovassili said negotiations with Greece's creditors -- the European Union, European Central Bank and International Monetary Fund -- would resume immediately after Thursday's vote.

The EU's Economic Affairs Commissioner Pierre Moscovici said they were aiming to finalise the deal by mid-August.

Both sides are under huge pressure to finalise the terms of the rescue before August 20, when Greece owes the ECB a loan repayment of 3.2 billion euros it cannot afford.