Voter lines were much shorter than when voting began a week ago, the observers said. At the same time, Egypt's election commission said turnout last week was much lower than it first reported.
Fifty-two percent of eligible voters turned out last week, not 62 percent, as the commission reported a few days ago. Authorities blamed the discrepancy on a mathematical error.
Egyptian authorities first estimated turnout at 70 percent.
The ruling military council had sought to use the high turnout figures as proof of voters' support for its legitimacy as custodian of the political transition, The New York Times reported.
Islamists won a 61 percent parliamentary majority last week -- figures that will "be more balanced" when the votes are finalized, independent presidential candidate Amr Moussa said.
The mainstream Muslim Brotherhood's Freedom and Justice Party received 36.6 percent and the ultraconservative Salafi Islamists Nour Party 24.4 percent.
Still, last week's results are "a message to the liberal forces that they have to come together and ... mobilize themselves in order to create a strong opposition within the Parliament," Moussa, a former Egyptian foreign minister and Arab League secretary-general, told CNN.
The Brotherhood movement, which seeks to expand Islamic law in many Middle Eastern countries, prevailed in two days of balloting that began a week ago. The balloting included voters in Cairo and Alexandria, cities where liberal parties said they hoped to exhibit their greatest strength.
The Brotherhood's party, which says it is moderate and emphasizes tolerance and pluralism, is accused by opposition leaders of using tactics similar to those of Mubarak to sway voters to its side, the Egyptian daily al-Ahram said Monday.
The party denies the accusations, saying any Supreme Electoral Commission violations, such as having party delegates in front of polling stations, were insignificant and were needed to facilitate the balloting by an overwhelming number of voters.
The commission said 9.7 million valid votes were cast last week.
The Nour Party, formed in January with the revolution that overthrew Mubarak, maintains an exacting version of Islam and seeks to impose strict Islamic law similar to that in Saudi Arabia, in which women must be veiled and alcohol is banned.
Islamists have formed governments in Tunisia and Morocco and are positioned for a major role in Libya after the ouster and killing of Moammar Gadhafi, the Times reported.
Egyptians cast three votes last week -- two for independent candidates and one for a party or coalition.
Four independent candidates secured seats but 52 other seats remained undecided because no candidate won a clear majority, leading to this week's runoff.
Two more voting rounds, one starting Dec. 15 and the second starting Jan. 3, are to take place in 18 of Egypt's 27 provinces for a total of 498 elected seats. Ten additional seats are to be appointed by the ruling military council.
The final election results for the lower chamber, known as the People's Assembly, are to be announced in January.
A three-round election for the upper house, known as the Shura Council, or the Consultative Council, is to follow, starting Jan. 29 and ending March 11.
The new Parliament was initially charged with selecting a 100-member committee to draft a new constitution. But the ruling military council that has run Egypt since Mubarak was ousted Feb. 11 has given contradictory indications about how much parliamentary input it will allow,. It has indicated it might choose 80 of the 100 committee members.
The new constitution would then to be submitted to a referendum, the council said.
Assuming it passes, a presidential election would be held by June 30, the council said.