CARACAS, Venezuela — Soldiers used tear gas, plastic bullets, and water cannons to scatter tens of thousands gathered yesterday to protest constitutional reforms allowing President Chavez to run for re-election indefinitely.

Led by university students, protesters chanted “Freedom! Freedom!” and warned that 69 amendments drafted by the Chavista-dominated National Assembly would violate civil liberties and derail democracy.

It was the biggest turnout against Chavez in months, and appeared to revive Venezuela’s languid opposition at a time when the president seems as strong as ever. Students promised more street demonstrations over the weekend, but no opposition-led protests were planned for Friday.

“This is a dictatorship masked as democracy,” a student, Jorge Rivas, 18, said. “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 during the melee. The local Globovision television network broadcast footage of several police beating an unarmed protester with billy clubs.

A student leader, Freddy Guevara, said it was not immediately 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, and a few lifted metal barricades and thrust them against police holding riot shields. Students retreated later when police fired plastic bullets. Rock-throwing clashes between students and Mr. Chavez supporters continued at a nearby university campus.

“Chavez wants to remain in power his entire life, and that’s not democracy,” a 20-year-old student who joined protesters marching to the National Elections Council, Gonzalo Rommer, said.

Deputy Justice Minister Tarek El Aissami blamed students for the violence, saying they forced their way through police barricades. But Vicente Diaz, one of the National Election Council’s five directors, accused National Guardsmen and police of using excessive force to disperse protesters. “We absolutely condemn the behavior of the authorities,” Mr. Diaz said.

The amendments 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 extend presidential terms from six to seven years while allowing Mr. Chavez to run again in 2012.