BEIRUT, Lebanon - Iraq ‘s Shiites owe their new power over the government to the U.S.-led invasion that toppled Saddam Hussein nearly four years ago. Many Lebanese Shiites would similarly like Israel ‘s summer war with Hezbollah to be the seed of their political ascendancy.

Now, filling the center of Beirut with daily rallies, Hezbollah is pressing for a larger say in the running of Lebanon and an end to the Shiites‘ history of being poor and oppressed.

However, granting Shiites more power could mean rearranging the delicate political balance struck to bring an end to Lebanon‘s 15 years of civil war in 1990. And Christians and Sunni Muslims — some of them deeply upset that Hezbollah provoked the summer war by capturing two Israeli soldiers on Israel‘s own territory — strongly oppose any such change.

And while Hezbollah has Christian allies on its side, the fight over Lebanon‘s future still is seen largely as a proxy battle between Iran , a Shiite theocracy backing Hezbollah, and the United States, which is supporting the government of Prime Minister Fuad Saniora, a Sunni.

"What we have in Lebanon now is a sectarian formula, and that is the real problem Lebanon is facing," said a senior Hezbollah official, Sheik Mohammed Kawtharani. "The pact (that ended the civil war) is like a time bomb."

To increase Shiite power, Hezbollah and its allies are demanding a veto-wielding share in a "national unity" government. On Dec. 1, they set out to force change, staging a massive sit-in outside government offices in Beirut and an open-ended series of demonstrations since.

Lebanon‘s Shiites have grown more assertive partly because of the new power of their fellow Shiites in Iraq. In the aftermath of last summer‘s war, too, Hezbollah became synonymous with Lebanon‘s Shiites more than ever. The other main Shiite group, the more secular and older Amal Movement, was largely eclipsed.

Hezbollah‘s opponents already have accused it of serving the interest its patrons, Iran and Syria , more than of Lebanon. "We don‘t want Lebanon to be an arena of the wars of others," Prime Minister Saniora said earlier this month.

Sateh Noureddine, a Shiite expert on Hezbollah and managing editor of Beirut‘s As-Safir newspaper, sees Hezbollah facing many challenges as the business of war becomes the more complicated business of politics.

At peace, Noureddine said, "Hezbollah is a much smaller force than it seems at times of war."