Opponents of Venezuelan President Nicolas Maduro have set up roadblocks and staged demonstrations demanding elections as the country's political and economic crisis deepens.
They responded with defiance to his call for a new constitution to end unrest that has killed 28 people.
Mr Maduro said his move was necessary to fend off a foreign-backed plot against him.
The US said it was a bid to cling to power, while Brazil called it a "coup".
The president's opponents want to hold a vote to remove him, blaming the left-wing president for food shortages that have led to rioting.
Mr Maduro has rejected their calls and issued a presidential decree creating a 500-member "constituent assembly" to rewrite the constitution, a step that would bypass the opposition-controlled Congress.
He announced the step to thousands of his supporters at a May Day rally on Monday.
"This constituent assembly that Maduro has announced is a manipulation to escape elections," Raul Hernandez told the AFP news agency.
He was among about 100 people blocking a major road in the capital Caracas.
Elsewhere, security forces deployed tear gas and a water cannon at anti-government demonstrators.
Opposition leaders have called for a "mega protest" on Wednesday.
"People, into the streets!" opposition leader Henrique Capriles said on Twitter. "You must disobey such lunacy!"
In Washington, US state department spokesman Michael Fitzpatrick told reporters: "We have deep concerns about the motivation for this constituent assembly which overrides the will of the Venezuelan people and further erodes Venezuelan democracy.
"What President Maduro is trying to do, yet again, is change the rules of the game."
Brazilian Foreign Minister Aloysio Nunes called the move a "coup".
"It's another step in breaking the democratic order, which contradicts the country's own constitution," he said.
Mr Maduro was elected in 2013 to succeed the late Hugo Chavez, a popular figure who introduced wide-ranging social welfare programmes.
But since then, falling prices for Venezuelan oil exports have cut government revenue and there have been shortages of food, baby milk, medicine and other basics.
The International Monetary Fund has forecast that inflation in Venezuela will be above 700 percent this year.
Presidential elections are due at the end of next year.