About 12,000 people took part in marches in Athens during a 24-hour nationwide strike - down from 50,000 in the biggest protest in May against austerity cuts imposed to secure a €110 billion bailout from the European Union and the IMF.
"Burn parliament!" a group of about 150 black-hooded youths shouted as they threw sticks, stones, bottles and petrol bombs at police guarding the building. Other marchers chanted "You are thieves" or "We don't owe you anything".
Police fired teargas to disperse protesters in the fifth joint strike by public and private sector unions this year.
At least three people were slightly hurt in brief, scattered clashes in Athens, during which some windows were smashed and bins set ablaze.
The march was otherwise largely peaceful and many businesses in the centre quickly reopened.
Greece's socialist government, which has 157 of the 300 seats in parliament, was yesterday beginng preliminary consideration of an overhaul of pensions.
It will raise women's retirement age from 60 to match men on 65 and require 40 years of social security contributions for a full pension.
The government says the reforms of the creaking system are vital to stave off bankruptcy for Greece, which has touched off a euro zone crisis with debts totalling 133 per cent of its annual output.
Tuesday's strike shut many public offices, banks and local media while hospitals operated with emergency staff.
About 50 domestic flights were cancelled but international flights were unaffected. Dozens of tourists were exasperated by the cancellation of some ferries to Greek islands. The Acropolis, the top tourist attraction in Athens, was open for visitors.
The unions said they would call another strike against the pension bill in July.
"The unions demand that parliament does not approve this monstrous bill," said Ilias Vrettakos, vice president of the public sector union Adedy. He said turnout was satisfactory given summer heat and weariness from past strikes.
In the big 5 May protest, three people were killed in the fire-bombing of an Athens bank. About 25,000 people turned out for the last similar strike on 20 May. Unions representing about 2.5 million workers, half the workforce, backed the strikes.
The repeated strikes and a rise in small bomb attacks since riots in 2008 have hurt tourism, which accounts for nearly a fifth of Greece's €240 billion economy. A senior official was killed last week by a bomb. In the morning, some ferries to islands were cancelled after strikers blocked boarding ramps in Piraeus, Athens' main sea port.
"They want to put us in a straitjacket so we work for free all our lives so that some can have their wealth and get very rich at our expense," said Sotiris Poulikogiannis, a protester in Piraeus.
"I am supposed to get married in Santorini, my family is coming and I can't get through," US tourist Kristin Shakavic said. "They are blocking us and I am so frustrated." Police said Piraeus generally operated as normal.