Ouagadougou (AFP) - Trade unions in Burkina Faso have called a general strike on Wednesday following a day of protests against long-serving President Blaise Compaore that saw hundreds of thousands of people on the streets.

Demonstrators wielding iron bars and stones battled police in the Burkina Faso capital on Tuesday after a massive rally against plans to extend the rule of the veteran president.

Firing tear gas, security forces charged the protesters after they ventured close to parliament in one of the strongest challenges to President Blaise Compaore in his 27 years in power.

The United States said it was "concerned by the spirit and intent" behind the draft bill to scrap the presidential term limits.

"We urge all involved, including Burkina Faso’s security forces, to adhere to non-violence, and to debate this issue in a peaceful and inclusive manner," State Department spokeswoman Jen Psaki said in a statement.

The violence erupted at the end of a march in Ouagadougou that drew up to a million people, according to opposition leaders fighting to prevent what they see as a constitutional coup by supporters of Compaore.

The demonstrators blew whistles and vuvuzela trumpets and carried banners reading "Blaise Get Out!" and "Don't Touch Article 37", in reference to the constitutional term limit that risks being scrapped to let the president seek re-election.

Police battled the stone-throwing demonstrators for hours and dismantled makeshift barricades put up by them to block traffic.

Trade unions and civil society groups called a general strike for Wednesday, hoping to keep up the pressure on Compaore.

The opposition has called for a blockade of parliament on Thursday when the legislature examines a proposed constitutional amendment that would allow Compaore to seek another term next year.

Schools and universities have closed for the week of protests planned in the impoverished West African nation.

Pre-dawn violence had already broken out early Tuesday as security forces fired tear gas at dozens of youths barricading the country's main highway, who hurled stones in response, according to an AFP reporter.

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Senior opposition figures Benewende Sankara and Ablasse Ouedraogo claimed a million-strong turnout. AFP reporters could not confirm that figure but estimated the crowd's number at well above a previous rally in August.

- 'This is our final warning' -

Several hundred people also occupied the Place de la Nation, a central square in the capital but later dispersed without incident.

"Our march is already a huge success, phenomenal," said opposition leader Zephirin Diabre. "Our struggle has entered its final phase. It's make or break time -- the nation or death!"

However, government spokesman Alain Edouard Traore late Tuesday issued a statement hailing the "vitality" of Burkina Faso's democracy despite what he termed as anti-government "misbehaviour".

Compaore was only 36 when he seized power in an 1987 coup in which his former friend and one of Africa's most loved leaders, Thomas Sankara, was ousted and assassinated.

The 63-year-old has remained in power since then, re-elected president four times since 1991 -- to two seven-year and two five-year terms.

In 2005, constitutional limits were introduced and Compaore is coming to the end of his second five-year term.

The opposition fears the new rules -- which are not expected to take previous terms into account -- would enable Compaore to seek re-election not just once, but three more times, paving the way for up to 15 more years in power.

The third largest party in parliament said at the weekend it would back the amendment, setting the ruling party on course to obtain the two-thirds majority it needs to make the change without resorting to a referendum as first promised.

Protesters have erected barricades and burned tyres in the capital since the proposal was announced on October 21.

Civil society groups have also asked for the project to be dropped, saying the country risks being paralysed if the amendment goes through.

Compaore's bid to cling on to power has angered the opposition and much of the public, including many young people in a country where 60 percent of the population is under 25.

Many have spent their entire lives under the leadership of one man and -- with the poor former French colony stagnating at 183rd out of 186 countries on the UN human development index -- many have had enough.