Tens of thousands of people packed into a stadium in Burkina Faso's capital Ouagadougou to voice their opposition to a referendum that would lift presidential term limits and allow the country's leader to seek re-election next year.
The rally on Saturday in the 45,000-seat Stade du 4 Aout was in response to a demonstration last month by President Blaise Compaore's supporters who backed the referendum.
"We don't want unlimited presidential mandates because Burkina Faso is not a kingdom where one dies in power," opposition leader Zephirin Diabre told the crowd.
"It is a republic where men move on and the institutions remain."
Compaore, who has held power since leading a coup in 1987, has positioned himself as a power broker in West Africa and a key ally of France and the United States in the fight against al Qaeda-linked militias in the region.
While he has not yet formally announced his plans, last month's rally was the clearest indication yet that he wants to
change the country's constitution.
The referendum would ask voters to approve changes to Article 37 of the constitution, which was drafted in 2000 and
limits presidents to two terms. Compaore secured his second five-year mandate in 2010.
"We must dissuade President Blaise Compaore and his supporters from organising the referendum. But if they persist,
we will move into another phase," said Diabre, who heads a grouping of 36 opposition parties.
"We will use all means allowed under the law to make them fail miserably," he said.
The opposition announced plans for another large rally in Bobo Dioulasso, the second largest city, on June 14, which would then be followed by demonstrations throughout the country.
Landlocked Burkina Faso is home to a growing gold mining sector but regularly grapples with poor harvests and food shortages. It remains one of the world's poorest nations.
Compaore secured 81 percent of the vote in the last election in 2010. But a brief army mutiny in 2011 rattled his otherwise firm grip on the military.
In January dozens of members of his party, including former close aides and key allies, quit to create a new movement to challenge him.