By Sam Cage and Radu Marinas

BUCHAREST (Reuters) - Romanian riot police fired tear gas at protesters who threw bricks and bottles back at them on Thursday in Bucharest's main square where demonstrations against the government's austerity drive have taken place since last week.

The police arrested up to 50 people believed to have triggered the violence including throwing metal fencing at them, according to private television station Realitatea TV.

Hundreds of demonstrators from an opposition rally which took place earlier in the day joined the protest in the main square in the evening. Traffic was blocked and protester numbers there rose to 1,500, riot police said.

About 7,000 opposition supporters rallied in Bucharest earlier on Thursday to demand the government's resignation, following a week of anti-austerity protests across the country which have sometimes turned violent.

The rally was organized by the leftist opposition USL to press for the resignation of Prime Minister Emil Boc's centrist coalition and his ally President Traian Basescu.

They paraded along a main boulevard from Bucharest's triumphal arch - modeled on the Arc de Triomphe in Paris - to the government headquarters, chanting "Down with Basescu" and comparing him with former communist dictator Nicolae Ceausescu.

"People are out on the streets in all the cities of Romania," USL co-leader Victor Ponta told the crowd, gathered in freezing temperatures.

"People are upset: many are angry because they lost their jobs, many are angry because their pensions were slashed, many others are humiliated by the rulers every day. But there is a single thing that unites us - we all want Basescu out."

The USL staged its march separately from main square protests which erupted spontaneously last week after a popular deputy minister, a respected Palestinian-born medic clashed with Basescu and resigned over a controversial health reform plan.

Hundreds of demonstrators supporting the spontaneous movement are meeting every day in the square, as well as other cities, to attack pay cuts and tax increases the government has implemented under an International Monetary Fund-led aid deal.

They have protested against the opposition as well as the government and there has been little violence since the weekend.

The austerity policies have helped Romania to put its once troubled finances on a steadier footing, but have also slowed recovery from a deep and painful recession

The USL has committed to stick with the IMF, but said it would revoke some of Boc's austerity measures if it takes power. It has called for a parliamentary election to be brought forward from late this year.

Latest opinion polls show support for the USL, a fragile leftist alliance, on about 50 percent compared with 18 percent for Boc's PDL.

Analysts say the protests are not big enough to sway policy but will make it even more difficult for the PDL to regain ground in the polls and hang on to power.

(Writing by Radu Marinas)