At least three people were killed and many injured as government forces in Nicaragua launched an operation to regain control of the city of Masaya.
Residents say security forces and militias used disproportionate force to clear barricades on Tuesday.
The operation was launched a day after the collapse of talks between President Daniel Ortega's government and the opposition.
More than 180 people have died in two months of anti-government protests.
Residents in Masaya say militias loyal to Mr Ortega were responsible for most of the violence.
They arrived in the early hours of Monday on tractors and lorries to regain control of the city.
Young protesters used homemade mortars and slingshots to repel them, Reuters news agency reports.
A former stronghold of the Sandinista Revolution in the 1970s, Masaya has now become a symbol of opposition to the Ortega administration.
The barricades were set up weeks ago in cities across Nicaragua by opposition activists, who are demanding the resignation of Mr Ortega.
The protests were triggered by government cuts to pensions and social security announced on 18 April.
The legislation, which has since been revoked, prompted nationwide demonstrations. Many people died in the marches – most of them university students – after a government crackdown.
The opposition now demand the resignation of Mr Ortega, a former Sandinista leader, and his wife, Vice-President Rosario Murillo.
Mr Ortega, who has been in office for 11 years, says he was democratically elected in 2016 with more than 70% of the vote and intends to serve out his term until 2021. The government accuses protesters of wanting to stage a coup d'etat.
A fresh attempt to find a negotiated solution to the crisis failed on Sunday.
Opposition delegates and mediators from Nicaragua's Roman Catholic Church walked out of negotiations, blaming the government for the collapse.
They accused the government of breaking its promise to invite international organisations to Nicaragua.
Both sides had reached an agreement on Friday to ask representatives of the Inter-American Commission on Human Rights (IACHR), the UN Commission on Human Rights and the European Union to come to Nicaragua to help investigate the deaths and acts of violence.
However, when the opposition Civic Alliance asked to see copies of the invitations on Monday, government representatives could not present them.
Foreign Minister Denis Moncada reportedly cited "bureaucratic" reasons for the failure to send out the invitations.