Thousands of Sunni demonstrators rallied Monday in Saddam Hussein's hometown of Tikrit (search) to denounce Iraq's new constitution a day after negotiators finished the new charter without the endorsement of Sunni Arabs.

Sunni leaders have urged their community to defeat the charter in a nationwide referendum Oct. 15, saying it had been rammed through the drafting committee by the dominant Shiite Arab and Kurdish alliance.

Iraq's biggest Sunni party, meanwhile, accused the Shiite-led security forces of massacring 36 Sunni men and dumping their bodies near the Iranian border. A major Shiite group said Sunni gunmen killed seven Shiite villagers after they refused to leave their homes.

The allegations were made one day after the Shiite and Kurdish-dominated government approved a draft constitution despite Sunni Arab objections and referred it to the voters in a referendum six weeks from now.

At least 2,000 protesters assembled in Tikrit near the office of the Association of Muslim Scholars (search) — a hard-line Sunni clerical group opposed to the U.S. occupation — carrying Iraqi flags and portraits of the former dictator.

"We sacrifice our souls and blood for you, Saddam," chanted the demonstrators. They carried pictures of Shiite clerics Muqtada al-Sadr (search) and Jawad al-Khalisi who have joined the Sunnis in opposing the constitutional draft.

Sheik Yahya Ibrahim al-Batawi, an organizer of the protest, read a statement denouncing the "Jewish constitution," saying its goal was to divide Iraq along sectarian and ethnic lines.

Both the constitutional squabble and the continued allegations of reprisal killings point to rising sectarian tensions in Iraq at a time when a growing number of Americans are questioning the Bush administration's policy in this country.

Tarek al-Hashimi, secretary-general of the Iraqi Islamic Party, told reporters that gunmen in police uniforms killed the 36 Sunni Arabs from Baghdad's Hurriyah neighborhood, shooting them and dumping the bodies in a dry riverbed last week near the Iranian border. Police found the bodies a few days later without an identification papers. All had been shot in the head, and some had their arms bound.

"The current government, especially the Interior Ministry, is responsible and must give a clear answer if it is involved in these acts of terrorism or is protecting terrorists or criminal groups that carried out such attacks," al-Hashimi said.

There was no comment from the Shiite-led Interior Ministry, but officials have denied similar allegations in the past. Shiites and Kurds dominate the U.S.-trained security services, and most of the insurgents are Sunni Arabs.

Meanwhile, the Supreme Council for the Islamic Revolution in Iraq, the main Shiite party, said the seven Shiites were killed in the farming village of Sayafiyah near Baghdad after Sunni gunmen ordered them to leave their homes.

Police said they had no information. The town is in an area south of the capital known as the "triangle of death," where Shiites are often targeted.

Sectarian tensions sharpened further after Sunni Arabs on the committee writing the new constitution refused Sunday to accept a draft approved Shiites and Kurds after months of difficult negotiations.

The absence of Sunni endorsement, after more than two months of intensive negotiations, raised fears of more violence and set the stage for a bitter political fight ahead of the referendum. A political battle threatened to sharpen communal divisions at a time when relations among the Shiites, Sunnis and Kurds appear to be worsening.

To encourage Sunnis to vote in the referendum, election authorities Monday postponed the deadline for voters to register by one week in the western province of Anbar, a rebellious Sunni stronghold where turnout in the January election was minimal.

Sunni negotiators delivered their rejection in a joint statement Sunday shortly after the draft was submitted to parliament. They branded the final version as "illegitimate" and asked the Arab League and the United Nations to intervene.

"We don't want to wage a war against anybody, but we say this draft has written in away that will divide and tear apart Iraq," Saleh al-Mutlaq, a top Sunni negotiator said Monday. "This constitution was written in a hurry and also passed in a hurry."

Al-Mutlaq said the Sunnis will try to bring down the constitution either through legal means or via the referendum.

"If the constitution is not changed, then we will try to bring it down either before the referendum through the law by filing a suit in international or local courts, if we can, challenging the legitimacy of this constitution and the National Assembly," al-Mutlaq told Al-Jazeera television.

In Baghdad, the secretary-general of the country's largest Sunni party told reporters the draft "does not represent our hopes and aspirations" and does not fulfill "our legitimate national principles."

"The draft has been submitted in the absence of the principle of compromise," said Tariq al-Hashimi of the Iraqi Islamic Party, which had representatives on the Sunni negotiating team.

The party said it would continue trying to amend the charter before it is presented to voters in October.

President Bush expressed disappointment the Sunnis did not sign on but pinned his hopes on the referendum.

But the depth of disillusionment over the charter in the Sunni establishment extended beyond the 15 negotiators, who were appointed to the constitutional committee in June under U.S. pressure.

The country's Sunni vice president, Ghazi al-Yawer, did not show up at a Sunday ceremony marking the completion of the document. When President Jalal Talabani said al-Yawer was ill, senior government officials howled with laughter.

Major deal-breaker issues included federalism, Iraq's identity in the Arab world and references to Saddam's Sunni-dominated Baath Party.

Sunnis fear federalism would lead to the breakup of the country into a Kurdish north and Shiite south, deprive Sunnis of Iraq's vast oil wealth concentrated at the opposite ends of the country, and open the door to Iranian influence in the Shiite south. Sunnis also wanted no reference to Saddam's party, fearing that would lead to widespread purges of Sunnis from government jobs.

Although Sunnis account for only 20 percent of Iraq's estimated 27 million people, they still can derail the constitution in the referendum due to a concession made to the Kurds in the 2004 interim constitution. If two-thirds of voters in any three provinces reject the charter, the constitution will be defeated. Sunnis have the majority in at least four provinces.

Defeat of the constitution would force new elections for a parliament that would begin the drafting process from scratch. If the constitution is approved, elections for a fully constitutional parliament will be in December.

In other developments on Monday:

— Two rockets slammed into the parking lot of the Oil Ministry building, wounding an employee and damaging several vehicles, police said.

— Unidentified gunmen shot and killed Brig. Gen. Numan Salman Faris, director of the district rapid response force in Baghdad's Azamiyah district.