Greek police fired teargas at rioters shouting burn parliament in Athens on Tuesday but fewer ordinary Greeks turned out for rallies in a sign of weariness with anti-austerity protests.

About 12,000 people joined marches in Athens during a 24-hour strike called by major unions against drastic pension reforms -- down from 50,000 in the biggest protest on May 5 and 25,000 in the latest similar demonstration on May 20.

Police used teargas against about 150 hooded protesters yelling Burn parliament! who threw sticks, stones, bottles and petrol bombs outside the building, where a committee of lawmakers later began debate about the pension reforms.

Parliament is expected to vote on them in coming weeks, and the socialist government is likely to be able to push them through.

Seven police officers were hurt and six demonstrators were arrested in scattered clashes. Some shop windows were smashed.

Analysts said factors such as summer heat, resignation after a spate of strikes and fears of violence may have kept many away from rallies against cuts imposed to win a 110 billion euro ($134.2 billion) bailout from the European Union and the IMF.

People are getting a little tired, there's a feeling of fatalism that things are going through as the government has no other choice and the opposition has no other proposals, said Theodore Couloumbis, deputy head of Greek think tank ELIAMEP.

Some were discouraged by fears of violence after three people died in the May 5 rally, when a bank was fire-bombed.

Ilias Vrettakos, a vice president of the main public sector union ADEDY, acknowledged weariness in the fifth strike this year by major public and private sector unions.

Participation...was satisfying, despite the weather conditions, the heat, and the fact that it was the fifth joint strike this year, he said.

Analysts say protests might flare up again in the autumn when cutbacks and tax hikes are felt more strongly, together with a projected rise in unemployment and deepening recession.

NEW STRIKE

Tuesday's strike shut many public offices, banks and local media, while hospitals operated with emergency staff. About 50 domestic flights were canceled but international flights were unaffected. Some ferries to Greek islands were canceled.

The Acropolis in Athens was open for visitors.

The unions said they would call another strike against the pension bill in July.

The socialist government controls 157 of 300 seats in parliament and is likely to be able to vote through the pension reform despite misgivings from within the party.

The bill will raise women's retirement age from 60 to match men on 65 and demand more years at work to qualify for a pension. The ruling socialists said that the existing system would mean more debt, already at 133 percent of GDP in 2010.

It was not a just system, it was not a viable system ...it was not an effective system, Ilias Mossialos, a Socialist lawmaker, said of the existing system during the pension debate.

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 euro ($297 billion) economy. A senior official was killed last week by a booby-trapped bomb.

In the main port at Piraeus, some tourists were exasperated at being stranded. I am supposed to get married in Santorini, my family is coming and I can't get through, U.S. tourist Kristin Shakavic said.

(Additional reporting by Tatiana Fragou and Gina Kalovyrna; Writing by Alister Doyle and Ingrid Melander; Editing by Mark Trevelyan)