Venezuela says it has closed parts of its border with Colombia to stop fraud in an campaign to force a referendum on President Hugo Chavez's rule.
The signature collection is taking place amid tight security
The move was designed to stop unregistered voters from crossing into Venezuela to sign a petition.
The opposition, which wants to remove the president in a referendum, says the closure is illegal.
The president's opponents have four days to gather the 2.4 million signatures needed for a vote.
The president's critics accuse him of authoritarian rule and mismanagement.
Vice-President Jose Vicente Rangel said the border closures were a response to intelligence reports that people with fake Venezuelan identity cards were trying to cross over from Colombia.
He said the move would not prevent the flow of trade between the two countries.
Members of Venezuela's opposition were quick to criticise the measure, describing it as illegal and unconstitutional.
They accuse the government of trying to prevent Venezuelans living in Colombia from signing the petition.
Earlier on Saturday, Venezuela's chief election official accused the military of slowing down the signing process.
"We have soldiers asking for identity cards, trying to manipulate materials and making decisions that go beyond their duty which is to guard the process," Ezequiel Zamora said.
But the head of the Organization of American States, Cesar Gaviria, who is in Venezuela observing the process, said 97% of the signature collection points had not reported any problems.
The Carter Center, overseen by former US President Jimmy Carter, is also overseeing the collection of signatures.
Large parts of the media, which are backing the opposition, reported a high-turn out on Friday.
Turn-out was allegedly high on Saturday but there was none of the party spirit of the previous day.
"I'm hoping all this will provide a peaceful way out of our country's crisis, with mutual respect for one another," one petition-signer Ismael Aris told AP news agency.
President Chavez has promised to leave office if he loses any referendum called but he has predicted that the chances of this happening are "almost zero".
Under the Venezuelan constitution, a president may be challenged after serving half of the six-year term.
Last September, the country's National Election Council rejected a petition signed by more than three million Venezuelans to call a referendum on Mr Chavez' rule.
The Council said the petition had been rejected because the signatures were gathered months before the mid-point of Mr Chavez's term in office.
The result of the petition campaign will not be known for several weeks.