Asif Ali Zardari and Nawaz Sharif meet to discuss concerns over vote tampering.
The widower of slain Pakistani opposition leader Benazir Bhutto met former premier Nawaz Sharif and said they discussed their fears that next week's elections would be rigged.
The meeting between Asif Ali Zardari and Sharif further cemented opposition to President Pervez Musharraf ahead of the vote on Monday, with opinion polls showing plummeting public support for the former army general.
"We have already expressed our reservations to the chief election commissioner about rigging in the general elections," Zardari told reporters after calling on Sharif at his house in the eastern city of Lahore.
"We want to make the constitutional institutions stronger so that no one could make them weaker."
His comments came after Human Rights Watch warned that Pakistan's Election Commission had failed to investigate reports of campaign violations, threatening the validity of the parliamentary elections.
The New York-based group said in a statement that the commission had ignored reports of arrests and harassment of opposition party members, and failed to act independently from Musharraf's administration.
Zardari - who is leading Bhutto's Pakistan People's Party (PPP) until the couple's son Bilawal is old enough to take the reins - said his party would decide whether to work with Musharraf if it won the election.
Sharif, who was ousted by Musharraf in 1999, ruled out any cooperation with the president.
Two surveys released Monday by US-based groups found that the PPP was the most popular in the country, with 50% of respondents in one poll saying they would vote for it.
Sharif's party was in second place in both polls, followed by Musharraf's allies in the former ruling Pakistan Muslim League-Q.
Zardari also visited the house of a senior lawyer and former PPP minister, Aitzaz Ahsan, who has been under house arrest since November, party aides said.
Ahsan defended the country's chief justice, Iftikhar Muhammad Chaudhry, in court when Musharraf tried to sack him last year. Musharraf deposed Chaudhry and put him, Ahsan and other lawyers in detention.