Bolivian President Evo Morales has formally received a new draft constitution, ahead of two referendums due on its reform measures.
"The people will never again be marginalized," the leader told crowds of supporters in La Paz.
He says the changes will give more powers to the poor, Indian majority.
But the country's three richest regions declared autonomy in protest at the plans for greater central control, with a fourth region expected to join them.
The lowland regions of Santa Cruz, Beni and Pando are home to the country's vital gas reserves. Political leaders there, and in Tarija, say the constitution is illegal because it was drawn up during an opposition boycott of parliament.
"Not all Bolivians have taken part in its drafting," said opposition deputy Lourdes Millares.
The new constitution will not come into effect unless it passed in two referendums.
President Morales, a left-winger elected in December 2005, is Bolivia's first indigenous president.
He made rewriting the constitution a key part of his reform agenda to give the indigenous majority greater political power, but the issue has deepened regional and ethnic divisions in the country.
He says the regions which are declaring autonomy are trying to split the country.
"We're not going to let anyone divide Bolivia," he told supporters on Saturday.
Meanwhile in Santa Cruz, a rally was held to mark the autonomy move, which local leaders plan to put to their own referendum.
"We're going to celebrate the birth of autonomous regions," said Santa Cruz politician Robert Gutierrez.
The region backed a statute on Thursday under which it would keep two-thirds of its tax revenues.
Pro-autonomy supporters object to the new constitution, which would allow consecutive five-year presidential terms, increase indigenous rights and redistribute wealth to the poorer highland areas of Bolivia.
The four regions backing autonomy are home to about 35% of the country's 9.5m population.
Extra army and police units were put on alert ahead of the weekend's marches, but the rallies passed off peacefully. ( BBC )