Police fired teargas at rock-throwing protesters after tens of thousands marched through Burkina Faso's capital calling for President Blaise Compaore, already in power for 27 years, to scrap plans to change term limits to stay in power.
An early morning march on Tuesday through the heart of Ouagadougou was peaceful but clashes erupted later as protesters tried to advance towards the National Assembly.
Tuesday marked the start of a civil disobedience campaign by opposition parties after the government asked the National Assembly to order a referendum on changing the constitution to let Compaore seek re-election next year rather than step down.
Former colonial power France, which uses Burkina Faso as a base for its Special Forces soldiers operating across West Africa, urged Compaore to abide by an African Union charter stipulating that leaders should not change the law to try to stay in power.
"The people have decided to start a general popular resistance. The first grievance is to get the withdrawal, pure and simple, of this legal project," Zephirin Diabre, head of the opposition delegation, told the crowd of demonstrators.
Protesters chanted "Step aside!" and "Don't touch Article 37", referring to the clause in the constitution that now bars Compaore, in power for 27 years, from running again next year.
Others carried banners comparing Compaore to Ebola, the virus that has killed nearly 5,000 people in the nearby states of Liberia, Guinea, and Sierra Leone.
"We must disinfect ourselves," one read.
There was no immediate figure for arrests or casualties, though Red Cross workers and emergency personnel took charge of some protesters injured during the demonstration.
Government sounds warning
In a statement issued late on Tuesday, the government congratulated the opposition leadership for what it said were largely peaceful demonstrations across the country. It said marchers in some towns had deviated from planned routes, however, leading to misbehaviour.
"The government calls upon the sense of responsibility and restraint in order to avoid any act that might compromise the peace and stability our country holds so dear," the statement said.
Protesters in Ouagadougou who marched towards the National Assembly, where the law will be debated on Thursday, were blocked by security forces who fired volleys of teargas and used water cannons.
They responded by burning tyres and throwing rocks. A pocket of demonstrators gathered in a downtown square pledging to hold out until Compaore shelved his plan, but were peacefully dispersed by police in the early evening.
"It's provocation. They [the authorities] want to set the country on fire," said a young protester. "Even if Blaise Compaore burns the country down, he will depart all the same."
Demonstrators in Bobo Dioulasso, Burkina Faso's second biggest town, 330km to the southwest of the capital, pulled down a statue of Compaore, a witness said.