Thousands of university students scuffled with police and government supporters during a protest Tuesday against constitutional reforms that would let President Hugo Chavez run for re-election indefinitely.

Police tossed tear gas canisters into the crowd of opposition students after bottle-throwing clashes broke out with a smaller group of pro-Chavez demonstrators near the National Assembly. Journalists estimated the crowd at about 20,000 protesters, but pro-Chavez lawmakers said it was much smaller.

The students said they fear civil liberties would be severely weakened under the constitutional changes, which would allow authorities to detain citizens without charges during a state of emergency.

"With this reform, the president is going to control everything. His power will be almost unlimited," said 22-year-old Adolfo Rengifo, who marched alongside other protesters blowing whistles and shouting "Reform, No! Democracy, Yes!"

The National Assembly, dominated by Chavez supporters, is poised to approve 67 constitutional amendments that would give the government control over the Central Bank, create new types of cooperative property and extend presidential terms from six to seven years while allowing Chavez to run again in 2012.

To take effect, the reforms must be approved by voters in a Dec. 2 referendum.

Protesters complained that police blocked their march before it reached the National Assembly while authorities frequently allow Chavez backers to stage street demonstrations without restrictions.

"It's clear proof of political discrimination," student leader Stalin Gonzalez said.

Public ombudsman German Mundarain — a close ally of Chavez — urged lawmakers on Monday to reconsider an amendment that would let the government waive some civil liberties during a state of emergency.

Roman Catholic leaders have argued that the proposed changes would violate fundamental rights.

Chavez, who was re-elected last year on promises of radical revolutionary change, says the reforms are fully democratic and necessary to accelerate Venezuela's transition to socialism.

"They say the reform is morally unacceptable. They are the ones who are morally unacceptable," Chavez said Sunday, referring to criticism by Catholic leaders.

Meanwhile, in an unusual form of political protest, Venezuelan media reported that headless effigies with messages reading "Reform, No!" and "Freedom" appeared hanging from street lamps on avenues in Caracas and the eastern city of Ciudad Guayana.