Hundreds of Venezuelan police and National Guard troops broke up a protest march on Friday with volleys of tear gas and blasts from water cannons that scattered a crowd of President Hugo Chavez's opponents.

Officials said about 20 people were treated for minor injuries, mostly for inhaling gas, while one police officer and a demonstrator suffered small cuts when they were hit by hurled objects. Some marchers were carried away after being overcome by tear gas.

People in surrounding buildings threw glass bottles at officers, and police responded throwing tear gas canisters at the buildings.

Some people stood in their windows and beat on pots and pans to protest the crackdown on the protesters. Others shouted "cowards" and "murderers" at the police.

Police broke up the demonstration as thousands of opponents and supporters of Chavez held separate May Day marches, bringing together labor groups and partisan demonstrators.

Officers in anti-riot gear pursued Chavez foes into side streets, including one group of several dozen who sat down in protest. Caracas Mayor Antonio Ledezma, a leading Chavez opponent, rose and raised a copy of the constitution above his head before police dispersed them by spraying water and tear gas.

"We've come here to defend the constitution," Ledezma told the crowd earlier.

The demonstrators had hoped to march to the National Assembly to protest new measures that have eroded the opposition mayor's power by stripping him of most of his budget and subordinating him to a city official appointed by Chavez.

Ledezma criticized the government for deploying hundreds of police officers and National Guard troops across the capital to guard against the march.

"The government wants to snatch away democracy with this escalation of authoritarianism," said Ledezma, who accused the president of violating the constitution by naming Jacqueline Faria, a vice president of his ruling party, above him to head the capital.

Chavez blamed the violence on the protesters and said authorities were obliged to disperse them with "one or two little cans of tear gas."

"We won't permit this violence," he told a large crowd of supporters. "That wasn't a march of workers, but a march of conspirators."

National Guard Gen. Alirio Ramirez defended the actions of security forces, saying authorities broke up the demonstration to maintain order. Police Chief Carlos Meza accused the opposition marchers of having a "violent plan."

Officials had denied protesters permission to march all the way to the National Assembly, and when they tried to pass a barricade erected by police, they were drenched with tear gas.

"They suffocated me with tear gas," said Sara Figueroa, a 56-year-old demonstrator who breathed oxygen through a mask at a tent set up by firefighters to treat the injured. Starting to cry, she said she felt "rage, impotence, like wanting to go kill those wretches."

In addition to supporting the mayor, marchers also expressed concern about other opposition leaders who have been sidelined by investigations into corruption allegations. Many carried signs picturing opposition leader Manuel Rosales, who faces a corruption charge in Venezuela but received political asylum in Peru earlier this week.

Elsewhere, a red-clad crowd of Chavez supporters paraded down a main avenue.

"I'm marching to support the president because I like my government," said Alfredo Lopez, a 72-year-old retiree. "I'm here to defend it."