BAGHDAD, Iraq, Feb. 12 — Shiite lawmakers chose Ibrahim al-Jaafari today to continue as prime minister in Iraq's next government, after a contentious internal ballot that exposed the growing power of anti-American fundamentalists within the new Iraqi Parliament.

The selection of Mr. Jaafari, a moderate Islamist, is the first step toward the creation of a full-term, four-year government. As the largest single bloc within the new 275-member Parliament, the Shiites have the right to choose a premier under Iraq's constitution. They will now begin negotiating in earnest with the leaders of Iraq's other political groups to create a cabinet.

That task, which is expected to take months, could be complicated by the surprise selection of Mr. Jaafari, 59, who has been widely criticized for his tenure as prime minister over the past year. Shiite leaders had expected him to cede his post to Adel Abdul Mahdi, an economist who is more popular with Kurdish and Sunni Arab leaders.

But in recent days followers of the radical Shiite cleric Moktada al-Sadr threw their support to Mr. Jaafari, who won 64 to 63 in a secret ballot this morning after efforts to reach a consensus failed. Mr. Sadr's followers now control the largest bloc of seats — 32 out of 130 — within the Shiite alliance. Mr. Sadr's followers decided to vote for Mr. Jaafari after he promised to implement their political program, said Bahaa al-Aaraji, a sitting member of Parliament and spokesman for the movement.

Mr. Aaraji would not say exactly what Mr. Jaafari had agreed to. But Mr. Sadr, who led two violent uprisings against the American occupation and interim Iraqi government in 2004, has made clear that he favors an American withdrawal from Iraq.

In recent visits to Iran and Syria, Mr. Sadr has expressed solidarity with the leaders of those countries and angry opposition to American policy toward them. Mr. Sadr also commands the Mahdi Army, a broad-based militia that has been mostly quiet since 2004, but is still armed and controls the streets in large areas of southern Iraq and eastern Baghdad.

Shiite leaders made a show of unity at a news conference after Mr. Jaafari's selection was announced, with Mr. Mahdi congratulating and praising his rival at a podium surrounded by banners bearing Shiite religious slogans.

"It's not important who won; the important thing is that the United Iraqi Alliance stays unified," Mr. Jaafari said.

A number of Iraqi leaders outside the Shiite alliance expressed surprise and concern about the election of Mr. Jaafari and the apparent influence of Mr. Sadr's followers.

"I was surprised and rather disappointed," said Adnan Pachachi, a former foreign minister and member of the secular alliance led by the former prime minister Ayad Allawi, which has 25 seats in the Parliament. "If he follows what the Sadrists want we will not be able to have a government of national unity."

Mr. Jaafari's government has been particularly unpopular with Sunni Arabs, who have accused the Interior Ministry of backing death squads that have rounded up and executed hundreds of Sunnis in Baghdad and elsewhere in recent months. Many Sunni Arabs have also accused Shiite militias, including Mr. Sadr's Mahdi Army, of playing roles in the killings.

"Jaafari failed to provide security and protect Iraqis," said Mahmoud al- Mashadani, a Sunni Arab and leading member of the Iraqi Accordance Front, which has 44 seats in the new Parliament. "Their business is not our business. But I expect trouble."

Talks on forming a cabinet are still in the earliest stages. But signs of tension were already apparent today, as Iraq's Kurdish president, Jalal Talabani — who has often sparred with Mr. Jaafari — angrily criticized Shiite leaders for trying to exclude Mr. Allawi's group, the Iraqis List, from the new government.

"We will never accept under any circumstances allowing the Iraqis List to be excluded from the next government," Mr. Talabani said, speaking at a news conference alongside the American ambassador to Iraq, Zalmay Khalilzad. "Those who reject the Iraqis List should understand that they are rejecting the Kurdish alliance as well: I say it loud and clear."

Some Shiite leaders said last month that they intended to form a three-part government with the main Kurdish and Sunni groups, relegating Mr. Allawi's secularists and some smaller parties to the opposition. But the mostly secular Kurds, who have 53 seats, appear to be vetoing that option.

Sunni Arab leaders also insist on a broader coalition. In the past week the Iraqi Accordance Front announced that it was teaming up with Mr. Allawi's group and a smaller Sunni Arab party to form a single negotiating bloc, the Joint Council for Patriotic Work, with 80 seats.

The task of accommodating all these groups within a single government will now fall — in large measure — to Mr. Jaafari. A physician with a quiet, apologetic manner, Mr. Jaafari fled Iraq in 1980 and spent years living in Iran, where he forged close ties with Iranian leaders, like many other members of his Dawa Party.

Since Mr. Jaafari became prime minister last year, suicide bombings and assassinations have surged, and oil production and exports have plummeted. Some Iraqi leaders, including fellow Shiites, have said he was not decisive enough for the job. Others said he allowed Shiite militias to infiltrate Iraq's police and military, leading to a rash of sectarian killings that have harmed efforts to draw Iraq's Sunni insurgents into the political process.