CARACAS, Oct 23 (Reuters) - Venezuelan students clashed with police firing tear gas on Tuesday as they marched to Congress to protest President Hugo Chavez's proposed constitutional changes to lift presidential term limits.

Chavez' plans have drawn criticism from opposition politicians, Roman Catholic Church authorities and student leaders, who say the left-wing former soldier is seeking to expand his power in the OPEC nation.

Thousands of marchers pushed through police lines in central Caracas, exchanging a volley of rocks and bottles with small groups of pro-Chavez demonstrators as police fired tear gas to disperse them.

Several people were slightly injured by rocks during the clashes, witnesses said.

"The message to the Congress and to the government is that there is ... a part of this country that rejects these reforms and we want to be heard," student leader Stalin Gonzalez told a local television station.

Chavez, a Cuba ally who has clashed with Washington over U.S. free market policies, says the changes are key to implementing a socialist revolution to help the poor majority with reforms such as extending social security benefits.

A student delegation delivered a document to Congress criticizing Chavez' proposal to overhaul the constitution as part of a drive toward a socialist state.

Marchers wore T-Shirts with the word "No" to symbolize their negative vote on an upcoming referendum to approve the constitutional changes, though pollsters say Chavez will likely win the election. He is popular for freely spending oil revenue on free health and education programs for the poor.

Lawmakers in Venezuela's National Assembly, where Chavez supporters hold all the seats, are rushing through a debate of the proposed constitutional rewrite to try to finished before the scheduled December referendum.

But even one pro-Chavez party, Podemos, has criticized the reforms, especially a proposal to lift limits on presidential terms that opponents fear will allow Chavez to stand indefinitely for re-election.

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