Hundreds of thousands of Venezuelans have protested against changes to the constitution proposed by the president.
Campaigners say they will do all they can to defeat the referendum
Venezuelans are due to vote on the proposals - which include the removal of presidential term limits - in a referendum on Sunday.
Critics accuse President Hugo Chavez of a power grab, but supporters say the changes will deepen democracy.
It is the latest in a series of student-led rallies, ahead of the "yes" campaign's final march on Friday.
However, correspondents say the "no" campaign is gaining force.
No official crowd estimates were available but an opposition politician put the figure at about 160,000.
Waving flags and banners and singing, the huge crowds of protesters staged a peaceful march through the capital's streets.
One female demonstrator told the BBC: "Although I don't think that all the things that President Chavez has done are bad... I don't like when the government imposes things."
Indefinite re-election of president, term increased from 6 to 7 years
Central Bank's autonomy ended
Structure of country's administrative districts reorganised
Maximum working day cut from 8 hours to 6
Voting age lowered from 18 to 16
Social security benefits extended to workers in informal sector
The students say they want the referendum postponed to give voters more time to study the plans.
However, the government has described the demonstrations as an opposition effort to destabilise the country ahead of the referendum on 2 December.
Earlier this month troops used tear gas and water cannon to disperse a rally, and last week gunmen opened fire on a peaceful protest march.
Students preparing to march from the Central University of Venezuela in Caracas were undeterred, and said they would do all they could to defeat the referendum.
A student leader, Stalin Gonzalez, told local media the show of opposition was to avoid "a continued polarisation of the country and a divided society".
If Hugo Chavez wins the referendum he will have the right to stand for re-election an unlimited number of times.
He has said he is prepared to serve for life as long as the people want it.
Student demonstrators clashed with police earlier this month
The BBC's James Ingham in Caracas says that is just one concern of opponents, who fear that in the future anyone disagreeing with the government will be penalised.
Other changes up for approval include giving the president control over the central bank, and the creation of new provinces governed by centrally appointed officials.
Mr Chavez is also proposing to bypass legal controls on the executive during a state of emergency, bring in a maximum six-hour working day and cut the voting age from 18 to 16.
A number of key defections from the president's camp have encouraged opponents - but Mr Chavez has dismissed these one-time allies as traitors.
Our correspondent says the president has made it personal, telling the populace that choosing "yes" is a vote for him and a "no" is a vote against him.