Masked gunmen seized three members of Iraq's largest Sunni Arab party and shot them to death in front of a mosque in northern Iraq, a day after gunmen elsewhere opened fire on Sunni leaders debating Iraq's constitutional process, injuring four people.

The violence came as followers of a radical Shiite cleric joined Sunni Arabs in protesting the inclusion of federalism in a new constitution, stepping up pressure on negotiators struggling to find a compromise just three days before a deadline to approve the charter.

With Sunni Arab negotiators still holding out against federalism and some other Shiite and Kurdish proposals, pressure of a different kind was mounting on Sunnis who support the constitutional process.

The masked gunmen seized the three Sunni Arab members of the Iraqi Islamic Party (search) in Mosul while they were hanging posters urging people to register to vote in a planned Oct. 15 referendum on the constitution that is being drafted.

The attackers later blocked a major road in front of Dhi al-Nourein Mosque, then brought the three out of cars and forced them to stand against a wall before spraying them with gunfire. The gunmen fled and the bodies were left behind.

The party has been urging Sunnis in recent weeks to register to vote in an October referendum on the new constitution. Many Sunnis boycotted the Jan. 30 elections following threats by insurgents seeking to derail the U.S.-backed political process and calls by clerics not to participate.

The shootings came a day after masked gunmen burst into the Sunni grand mosque in the tense city of Ramadi as religious, political, and tribal leaders met to discuss possible Sunni participation in the constitutional process. The gunmen asked participants to end their meeting, then opened fire on them, said Omar Seri, secretary of the governor of Anbar province.

Three members of the Sunni Association of Muslim Scholars (search) and a bodyguard were injured, Seri said.

Negotiators from the biggest Shiite party have been pressing for federalism, hoping to create an autonomous region in areas they dominate in central and southern Iraq as Kurds have in the north — both areas rich in oil.

But they also face opposition from within the majority religious community, including from followers of radical Shiite cleric Muqtada al-Sadr (search) who staged anti-federalism protests in several districts of Baghdad.

In mostly Shiite Sadr City, about 1,000 people demonstrated waving Iraqi and Shiite flags and chanting "No to separation, yes to unity."

"We reject federalism under occupation. Federalism does not represent our people's aspirations," said Abdelzahra al-Sueidi, an al-Sadr aide.

In Kazimiyah — another Shiite bastion — hundreds of al-Sadr supporters turned out at a protest, flying national flags and holding up banners reading "We want a united and stable Iraq."

"After all, we are one united people whether we are Sunnis or Shiites, Kurds or Arabs," Hazim al-Aaraji, another al-Sadr aide, told the congregation in a Kazimiyah mosque during Friday prayers.

More than 1,000 people also rallied in Baqouba, northeast of Baghdad, to protest the proposed constitution. The demonstrators chanted slogans against the proposed division of Iraq.

U.S. officials are eager for the Iraqis to agree on a draft constitution by the new Monday deadline after they failed to strike a deal by the original Aug. 15 date.

The United States believes a constitution would be a major step in the political transformation of Iraq and would help lure disaffected Sunni Arabs away from the Sunni-dominated insurgency. If parliament signs off on the draft, it will go to the voters for ratification in a referendum Oct. 15.

But passions are running so high that a charter might sharpen sectarian and ethnic differences here, complicating political compromise. Talks were under way Friday in the heavily guarded Green Zone of central Baghdad. If the factions do not agree by the new deadline, parliament must be dissolved.

In Thursday's constitutional deliberations, Sunni Arab members of the drafting committee met with al-Jaafari to present their objections to federalism and other issues blocking an agreement.

Afterward, leaders of the factions — Shiites, Sunni Arabs and Kurds — conferred late into the night at the home of Vice President Adil Abdul-Mahdi.

Haitham al-Husseini, a spokesman for the Supreme Council for the Islamic Revolution in Iraq (search) — the biggest Shiite party — said he expected a breakthrough within 48 hours.

Parliament voted unanimously last Monday to extend the deadline by one week after negotiations deadlocked over a number of issues. In addition to federalism, stumbling blocks included Kurdish demands for the right to secede, distribution of oil wealth and the role of Islam.

Mahmoud Othman, a Kurdish lawmaker, said all sides were determined to finish the constitution on time "but the question is, would this draft satisfy the Sunni Arabs' demands."

In violence Friday:

— A roadside bomb detonated near a U.S. military convoy in the capital, injuring one civilian but causing no U.S. casualties.

— Insurgents assassinated a city council member in the northern city of Hawija. Police said the councilman, Aswad Omar Nayef, on his way to Kirkuk when he was ambushed by gunmen. Insurgents have killed dozens of government and local officials.