Led by university students, demonstrators chanted "Freedom! Freedom!" and warned that 69 amendments drafted by Venezuela's Chavista-dominated National Assembly would violate civil liberties and derail democracy.

"This is a dictatorship masked as democracy," said Jorge Rivas, an 18-year-old student. "Chavez wants our country to be like Cuba, and we're not going to allow that to occur."

Authorities broke up the protest outside the headquarters of the country's electoral council, reporting that six police officers and one student were injured.

But students said dozens of protesters were hurt in the melee. The Globovision television network broadcast footage of several police beating an unarmed protester with billy clubs.

Student leader Freddy Guevara said it was not clear how many students were arrested, and he urged local human rights groups to help verify the number of detained protesters.

Students hurled rocks and bottles, lifting sections of metal barricades that a few thrust against riot police. They retreated when police fired plastic bullets in their direction. Later, clashes between rock-throwing students and Chavez supporters carried on at a nearby university campus.

"Chavez wants to remain in power his entire life, and that's not democracy," said Gonzalo Rommer, a 20-year-old university student who joined protesters as they marched to the National Electoral Council downtown.

At issue are constitutional amendments that would give the government control over the Central Bank, create new types of cooperative property, allow authorities to detain citizens without charge during a state of emergency, and abolish presidential term limits, allowing Chavez to run again in 2012.

To take effect, the reforms must be approved by voters in a Dec. 2 referendum. Lawmakers are expected to give final approval to the amendments on Friday during a special congressional session.

Chavez -- a close ally of Cuban leader Fidel Castro -- denies the reforms threaten freedoms. The changes, he says, will help Venezuela advance toward socialism and give neighborhood-based assemblies more discretion to use government funds on local projects.

Opposition parties, human rights groups and representatives of the Roman Catholic Church fear the changes would severely curtail civil liberties.

Thursday's protest was the largest anti-Chavez demonstration in recent months, and it appeared to revive Venezuela's languid opposition movement at a time when Chavez seems as strong as ever. Students promised more street demonstrations over the weekend, but no opposition-led protests were planned for Friday.

Deputy Justice Minister Tarek El Aissami blamed students for the unrest, saying they'd forced their way through police barricades. But Vicente Diaz, one of the electoral council's five directors, criticized National Guardsmen and police for using excessive force to break up the march.

"We absolutely condemn the behavior of the authorities," Diaz said.