A student dressed as captain Jack Sparrow performs in front of riot policemen during a rally in Santiago June 16, 2011. Students, teachers and workers staged a demonstration on the streets demanding changes in the public state education system. [Photo/Agencies]

SANTIAGO - Defying police water canon and tear gas, tens of thousands of students marched in Chile's capital and main cities on Thursday, the latest in a series of protests seen obstructing the government's legislative agenda.

Carrying banners that read "Don't profit" (from public education), students demanded improved education standards, lower university fees and cheaper bus passes from Chile's center-right government.

Television images showed some protesters, wearing T-shirts over their faces, throwing rocks at armored police vans that spewed tear gas in the capital Santiago. Buses were covered in student slogan graffiti.

Most of the protesters -- estimated by police at around 70,000 in the capital alone -- marched peacefully in one of the biggest rallies since President Sebastian Pinera took office last year, ending 20 years of center-left rule in Chile.

Student protests have plagued successive governments in a country where students must pay towards their state education, and are piling fresh pressure on the embattled administration.

Pinera's approval rating hit a new low of 36 percent in May, a survey by pollster Adimark GfK showed this month, punished for his government's approval of a controversial hydro electric project and despite the strong economic recovery from the ravages of a devastating earthquake last year.

A billionaire, Pinera has also been buffeted by a scandal over credit irregularities at retailer La Polar that have sent shockwaves through the stock market.

Critics say the government failed on oversight.

"This makes it very difficult for the government to advance its legislative agenda," said Patricio Navia, a political scientist at New York University.

"People are not paying attention to what happens in Congress, so the political upheaval in the streets and the La Polar issue too make it very difficult for the government to focus on its legislative agenda."

HAMSTRUNG IN CONGRESS

The government will likely struggle to push through a plan to eliminate a 7 percent health contribution by pensioners, post-natal legislation and, ironically, education reforms that are being negotiated in Congress.

However key capital market reforms in the world's top copper producer, aimed at boosting liquidity and ensuring the solvency of the country's financial markets, are less divisive, and seen to be safe.

The government is sending components of that reform package to Congress as separate bills, hoping it will speed up their passage. It has already submitted bills that regulate derivatives, as well as the use of insurance contracts.

"We want the government to listen to the demands students have had for years," said student Marco Caceres as he marched in the downtown Santiago, as others blew whistles and shouted.

Conservative Pinera took power in March 2010, pushing a raft of labor, health, energy, electoral and environmental reforms, although his first year was consumed with reconstruction after a massive quake in February 2010.

Analysts say many Chileans feel he has failed to deliver on pledges to reduce poverty and raise living standards.

However Chile's center-left opposition is in disarray after losing last year's presidential election, and its own approval rating is far lower than Pinera's, at just 23 percent.

"The street protests are filling the vacuum left by the fact there is no institutional opposition," said political analyst Ricardo Israel.