Protesters thronged the streets of Caracas city centre
Huge crowds of Venezuelans have taken to the streets to oppose an attempt by President Hugo Chavez to scrap rules barring him from re-election.
His supporters say he needs to stand for a third term to finish his socialist revolution, but his opponents worry that it would erode democracy.
The country will vote in a referendum on the issue next Sunday.
Local media said 600,000 people turned out for the "no" campaign's march, but other sources said tens of thousands.
Mr Chavez, who was rallying support for a "yes" vote in other areas of Caracas, joked that he would have attracted "100 times" the level of support received by the "no" campaign.
But the BBC's Will Grant, in Caracas, says the protest underlined the fact that there is strong opposition to the constitutional change proposed by Mr Chavez.
The president can usually rely on support from poorer sections of society, but our correspondent says that opposition to this move comes from many different sectors of Venezuelan society.
Voters rejected a similar proposal in 2007.
Even so, opinion polls suggest that Mr Chavez's "yes" campaign is heading for a narrow victory in next week's referendum.
This would allow elected officials, including the president, to seek indefinite re-election.
Mr Chavez recently celebrated 10 years in power, but under current rules he would have to step down in 2012.