Islamists claimed a decisive victory on Wednesday as early election results put them on track to win a dominant majority in Egypt’s first Parliament since the ouster of Hosni Mubarak, the most significant step yet in the religious movement’s rise since the start of the Arab Spring.

The party formed by the Muslim Brotherhood, Egypt’s mainstream Islamist group, appeared to have taken about 40 percent of the vote, as expected. But a big surprise was the strong showing of ultraconservative Islamists, called Salafis, many of whom see most popular entertainment as sinful and reject women’s participation in voting or public life.

Analysts in the state-run news media said early returns indicated that Salafi groups could take as much as a quarter of the vote, giving the two groups of Islamists combined control of nearly 65 percent of the parliamentary seats.

That victory came at the expense of the liberal parties and youth activists who set off the revolution, affirming their fears that they would be unable to compete with Islamists who emerged from the Mubarak years organized and with an established following. Poorly organized and internally divided, the liberal parties could not compete with Islamists disciplined by decades as the sole opposition to Mr. Mubarak. “We were washed out,” said Shady el-Ghazaly Harb, one of the most politically active of the group.

Although this week’s voting took place in only a third of Egypt’s provinces, they included some of the nation’s most liberal precincts — like Cairo, Port Said and the Red Sea coast — suggesting that the Islamist wave is likely to grow stronger as the voting moves into more conservative rural areas in the coming months. (Alexandria, a conservative stronghold, also has voted.)

The preliminary results extend the rising influence of Islamists across a region where they were once outlawed and oppressed by autocrats aligned with the West. Islamists have formed governments in Tunisia and Morocco. They are positioned for a major role in post-Qaddafi Libya as well. But it is the victory in Egypt — the largest and once the most influential Arab state, an American ally considered a linchpin of regional stability — that has the potential to upend the established order across the Middle East.

Islamist leaders, many jailed for years under Mr. Mubarak, were exultant. “We abide by the rules of democracy, and accept the will of the people,” Essam el-Erian, a leader of the Brotherhood’s new party, wrote in the British newspaper The Guardian. “There will be winners and losers. But the real — and only — victor is Egypt.”

Results will not be final until January, after two more rounds of voting. And the ultimate scope of the new Parliament’s power remains unclear because Egypt has remained under military rule since Mr. Mubarak resigned as president in February. But Parliament is expected to play a role in drafting a new Constitution with the ruling military council, although the council has given contradictory indications about how much parliamentary input it will allow.

The emergence of a strong Islamist bloc in Parliament is already quickening a showdown with the military. Brotherhood leaders announced Wednesday that they expected the Islamist parliamentary majority to name a prime minister to replace the civilian government now serving the military. In response, a senior official of the military-led government insisted that the ruling generals would retain that prerogative.

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