Mr Morales wants more power for Bolivia's indigenous people
Bolivian President Evo Morales has joined the start of a 200km (120-mile) march to push congress to call a referendum on a new draft constitution.
Thousands of farmers and union members have begun marching to the administrative capital, La Paz.
Mr Morales says the new constitution will give greater power to the country's majority indigenous people.
Opponents, based in the gas-rich lowland provinces, want greater autonomy for their regions.
The new constitution must be approved in a national referendum - which only congress can schedule.
Mr Morales' Movement Toward Socialism party controls the lower house of congress but the opposition controls the senate.
The new constitution would enshrine the president's nationalisation and land redistribution programmes.
"This march is about refounding Bolivia," Mr Morales said before the marchers set off from the mountain town of Caracollo.
They are expected to reach La Paz in about a week, by which time Mr Morales said there would be "more than a million" marchers.
Mr Morales is Bolivia's first indigenous president. He was elected in 2005 with huge support from the indigenous community.
The new constitution would give more power to the country's indigenous majority, set a limit on large landholdings, and give the state more control over the economy.
The plan has met with sharp opposition in Bolivia's richer lowland provinces. Four of them have declared autonomy in protest.
In September, tensions over the constitution erupted into violence, with 30 people killed in the northern province of Pando.