The showing in Egypt - long considered a linchpin of regional stability - could signal that parties and candidates connected to political Islam are the main beneficiaries of this year's Arab Spring uprisings.
Tunisia and Morocco have elected Islamist majorities to parliament, and Islamist groups have emerged as a strong force in Libya since rebels overthrew Moammar Gadhafi in August. Islamists also play a strong opposition role in Yemen.
Judges overseeing the Egyptian vote count said Thursday that near-complete results show the Muslim Brotherhood, the country's largest and best-organized political group, could take as much as 45 percent of the contested seats.
Parties backed by ultraconservative Salafist Muslims looked poised to take 20 percent, giving Islamist parties a striking majority in the first round of voting in key districts, including Cairo and Alexandria.
Similar results in the remaining rounds would give Islamist parties a majority in parliament, which many believe they will use to steer the long-secular U.S. ally in a more religiously conservative direction.
The Islamist victories came at the expense of a coalition of liberal parties called the Egyptian bloc, the group most closely linked to the youth activists who launched the anti-Mubarak uprising. It is expected to win only about 20 percent of seats.
The election also provided an opening for the Salafist Muslims, whose strict practice is similar to that in Saudi Arabia. While the Muslim Brotherhood has said it would preserve individual rights, Salafi groups are not shy about their ambition to turn Egypt into a state where women must dress modestly and TV content deemed offensive will be banned.
During the campaign, the Brotherhood's leadership often avoided strict Islamist rhetoric in favor of more inclusive messages about social equality and clean government.
Critics, however, worry that once in power, the group will band together with its Islamist allies to impose stricter social codes. Many in Egypt's Coptic Christian minority fear they will face new restrictions on building churches.
The Obama administration has lauded the elections, saying it would cooperate with the victors, no matter their persuasion.
Israel, which has long considered its peace treaty with Egypt a buffer against regional war, worries that Islamists will be less cooperative than Mubarak was. Israel is highly unpopular in most of Egyptian society, and Brotherhood leaders have suggested they would review Egypt's relationship with the Jewish state.