The three-day meeting organised to scuttle the charter concluded with a communique on Saturday urging a no vote "if the constitution's main points on Iraq's unity and Arab identity are not rectified, as well as articles related to political and racial segregation".
Meeting organiser and prominent cleric Shaikh Abdul-Latif Himayem said he expected at least 51% of Iraq's electorate to vote against the charter in the 15 October referendum.
"We are calling on all leaders, clergymen, tribal chiefs and university professors to mobilise their constituents to go to the polls and to vote no to the constitution," Himayem said.
"We have also prepared a petition and we expect to gather support from about 5 million people from all over Iraq to say no to the constitution."
The conference was held in the Jordanian capital, Amman, for security reasons.
Iraq's Sunni Arabs have strongly opposed the draft constitution, largely because they say it would give Shia in the south the right to form a mini-state that Sunnis fear will deprive them of the area's oil wealth and lead to Iraq's fragmentation.
But the constitution got a major boost this week when it won support from Iraq's most revered Shia leader, Grand Ayat Allah Ali al-Sistani, as well as Abdul Aziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq, Iraq's largest Shia party and the senior partner in the ruling coalition.
Iraq's Shia, whose solidarity is essential if the constitution is to pass, make up 60% of the population. Sunni Arabs and Kurds account for 30%-40%.
Al-Hakim also accused groups he called "terrorists and remnants of the former regime" of seeking to disrupt the referendum process to prevent the Iraqi people from casting a yes vote.
"We are calling on all leaders, clergymen, tribal chiefs and university professors to mobilise their constituents to go to the polls and to vote no to the constitution"
Shaikh Abdul-Latif Himayem, meeting organiser and cleric
If two-thirds of the voters in any three of Iraq's 18 provinces reject the document, parliament must be dissolved and a general election held for a new chamber that would undertake the drafting of a new constitution.
The Sunnis are the majority in three Iraqi provinces. However, all three are among the hardest hit by violence and getting out the vote on 15 October could prove difficult.
The 150 leaders from Iraq's violence-torn Sunni heartland who gathered in Amman are concerned that the proposed constitution threatens Iraq's unity.
The charter "has articles which could lead to Iraq's division and eradicate Iraq's Islamic and Arab identity", said Hamid Rashid al-Mhanna, head of the Albu Alwan tribe, one of the largest tribes in Anbar province.
Sunnis say they fear the charter
will lead to the breakup of Iraq
Delegate Faris Taha al-Faris called the constitution "a plot against Iraq" and complained that it was largely written by Americans.
"We call on the Shia in the south, the Kurds in the north to also reject it because there are many people who do not agree with this charter," he said.
The communique issued at the end of the conference urged the Iraqi National Assembly and the interim government to preserve Iraq's territorial integrity.
It asked that the referendum be carried out under the supervision of the United Nations, the Arab League, the Organisation of the Islamic Conference and the European Parliament.
It also urged for "polling centres to be protected and the appropriate atmosphere provided for voters by stopping all military operations".
"Many of the moderates will turn to the resistance because it is a legitimate right to fight this constitution"
Shaikh Abdul-Latif Himayem
Sunnis have increasingly complained of being hard hit in the crackdown by US and Iraqi forces against fighters - most of whom are believed to come from the Sunni Arab population.
US and Iraqi troops have launched full-scale assaults on a number of Sunni cities to root out fighters.
The Sunni delegates threatened to declare "civil disobedience" if the military operations against Sunni cities continued.
Himayem, the cleric, emphasised that violence would increase if the constitution passes.
"Many of the moderates will turn to the resistance because it is a legitimate right to fight this constitution," he warned.