The activists said they suspected the Egyptian government had played a role in the disappearance of the pages, possibly by covertly bombarding Facebook with complaints about the pages that resulted in their removal, but offered no evidence to support the allegation.
Egyptian interior and information ministry officials were unavailable for comment on Friday.
Facebook said in an email message that its security systems, designed to protect users on the site, had led it to remove the pages.
"We have been working with political and human rights groups both inside and outside of Egypt since this was brought to our attention to explain the measures they need to take to keep their accounts and associated activity within the site rules," said Stefano Hesse, Facebook head of communications for Europe, Middle East and Africa Stefano Hesse said.
"This has led to the two large pages that were of most concern being reinstated," he added.
The web is one of the few public platforms for dissident voices in a country where an emergency law in place since 1981 makes political activism a challenge by hampering efforts to establish a popular, united opposition movement.
"It is strange that the two biggest Facebook pages in Egypt and the Arab world are all of a sudden deleted," one activist, who ran one of the pages but did not want to be named, told Reuters.
One page, called "We are all Khaled Said" and with 330,000 registered users, reappeared 15 hours later after Egyptians living abroad pressed Facebook's administrators to reinstate it, the page's creators said.
The second, called "Mohamed ElBaradei" after the former U.N. nuclear watchdog chief, an Egyptian native who led a now-fizzled constitutional reform campaign, resurfaced later on Friday. That page boasted 298,000 users.
Khaled Said was a web activist who human rights groups say was killed as a result of police brutality but state authorities say died by choking on drugs. It is Egypt's largest such page.
Facebook campaigns played a key role in galvanising protests in 2008 against rising prices and low wages that led to clashes with police in the city of Mahalla el-Kubra.
Sunday's parliamentary vote is widely expected to produce a routine victory of President Hosni Mubarak's National Democratic Party, following a state-run crackdown on media in recent weeks.
The government has shut 12 television stations and forced a number of government critics off the air, saying the channels aired content that violated their permits.
Its founders told Reuters the Khaled Said page had called for a day of anger on Friday to commemorate the death of another Egyptian, Ahmed Shabaan, 19, whose body was found in a canal in Alexandria last week.
"Facebook decided to close the page after the arrival of a large number of complaints from sources who want to silence our voice," one of its founders said.