Protesters angry at plans to allow Burkina Faso's President Blaise Compaore to extend his 27-year-rule have set fire to parliament.
Correspondents say the city hall and ruling party headquarters are also in flames in the capital, Ouagadougou.
A huge crowd is surging towards the presidential palace and the main airport has been shut.
MPs have suspended a vote on changing the constitution to allow Mr Compaore to stand for re-election next year.
Five people have been killed in the protests, among the most serious against Mr Compaore's rule, reports BBC Afrique's Yacouba Ouedraogo from the capital.
The military fired live bullets as protesters stormed parliament, our correspondent says.
Journalists are now gathered outside the defence ministry awaiting a statement from the military, he says.
Witnesses say dozens of soldiers have joined the protests, including a former defence minister, Gen Kouame Lougue.
The main opposition leader, Zephirin Diabre, has called on the military to side with "the people" and has demanded the resignation of the president.
Mr Compaore's whereabouts are unknown, but he has appealed for calm via Twitter.
He first took power in a coup in 1987, and has won four disputed elections since then.
At the scene: Joost Laane, speaking to BBC Focus on Africa
I am in an area where many MPs live and I have seen two of their homes set ablaze and smoke coming out of another two or three homes. Hotel Azalai, one of the main hotels in the city, is also on fire.
Two helicopters flew over my house - the president's and a normal helicopter. I cannot confirm whether the president was in one of them.
No-one knows what is going to happen next. It is chaotic and tense. We hear sporadic gunfire.
There is no TV anymore. So we are depending on internet access and phone calls. The 3G network and the texting system are blocked.
The opposition has called for a campaign of civil disobedience to demand that he steps down in elections next year.
"October 30 is Burkina Faso's black spring, like the Arab Spring," opposition activist Emile Pargui Pare told AFP news agency.
State television went off air after protesters stormed the building housing it and ransacked it, Reuters quotes a witness as saying.
About 1,500 people breached the security cordon at parliament, according to AFP.
Protesters were setting fire to documents and stealing computer equipment and cars outside the building were also set on fire, the agency says.
- Served under President Thomas Sankara as minister of state to the presidency
- Took power after Sankara was killed in mysterious circumstances by a group of soldiers in 1987
- First elected president in 1991 and again in 1998
- A new constitution in 2000 limited presidents to two terms in office, and limited terms to five years
- Won two further terms
- Protests at attempts to amend the term limits began a year ago, fuelled by the high cost of living
A massive crowd has also converged on the main square in Ouagadougou and is marching towards the presidential palace, which is about 5km (three miles) away, our reporter says.
A government helicopter flying overhead was firing tear gas at them, Reuters reports.
There are also reports of protests in the south-western city of Bobo Dioulasso.
The government has been forced to suspend Thursday's parliamentary vote on a constitutional amendment that would have lifted the limit on presidential terms so that Mr Compaore could run for office again in 2015.
It is not clear whether the government intends to hold the vote at a later stage, correspondents say.
Mr Compaore is a staunch ally of the US and France, which uses Burkina Faso as a base for military operations against militant Islamists in the Sahel region.
Both France and the European Union (EU) have called on him to scrap the proposed constitutional amendment.
The EU said it could jeopardise Burkina Faso's stability. The US has also raised concern about the proposed amendment.