Creditors have demanded the measures before they give Greece more funds from a $152 billion package of bailout loans from other eurozone countries and the International Monetary Fund. Greece says it will run out of money in mid-November without the $11 billion installment.
But Greeks said they already were reeling from more than 11/2 years of austerity measures.
"We just can't take it anymore. There is desperation, anger and bitterness," said Nikos Anastasopoulos, head of a workers' union, as he joined the demonstration early in the day.
The bill won initial approval in the 300-member Parliament late Wednesday, with 154 deputies voting in favor on principle and 141 against. A second vote, on the bill's articles, is due Thursday.
Hours before the vote, one of Athens' largest demonstrations in years degenerated into violence.
Police responded with tear gas and stun grenades. Authorities said 50 police were injured in the clashes, along with at least three demonstrators, while 33 people were detained for questioning or arrested for alleged involvement in the rioting.
The new measures have even prompted some lawmakers from the governing Socialists to threaten not to vote for at least some of the articles in the bill. But Finance Minister Evangelos Venizelos insisted there was no choice.
"We have to explain to all these indignant people who see their lives changing that what the country is experiencing is not the worst stage of the crisis," he said in Parliament. "It is an anguished and necessary effort to avoid the ultimate, deepest, and harshest level of the crisis. The difference between a difficult situation and a catastrophe is immense."