"This constitution bears in it the germs of Iraq's division, the loss of its Arab identity and the plundering of its national wealth," said a joint statement on Saturday.
"As a result, we call for all Iraqis to reject this constitution by all legitimate means," said the statement from groups including the influential Association of Muslim Scholars and the main Iraqi Islamic party.
The head of the Association of Muslim Scholars, Harith al-Dhari, emphasised that the coalition urged a rejection, not a boycott, of the referendum.
Sunni Arabs account for approximately a fifth of Iraq's population, but many reject the constitution because of federal provisions they fear will lead to a breakup of the country and an unfair distribution of oil wealth.
The constitution will have to be rewritten if more than two-thirds of voters in at least three of Iraq's 18 provinces vote no, something Sunni Arabs are thought to be able to muster.
Sectarian war rejected
The leader of the Sunni High Committee for Dawa, Irshad and Fatwa (call, guide and religious decree) earlier also called for a "no" vote while rejecting an al-Qaida call for all-out war against Shia.
"We reject the constitution because it doesn't bring about equality among Iraqis," Shaikh Zakaria Tamimi said.
An Iraqi worker prepares copies
of the draft constitution
Tamimi rejected a call made last month by al-Qaida frontman in Iraq Abu Musab al-Zarqawi for all-out war against the country's Shia population.
"Zarqawi has no religious authority, and the Salafists feel that you can't spill the blood of Muslims," he said.
"Shiites are not apostate, and declaring war on Shiites is unacceptable," he said, accusing the Jordanian-born al-Zarqawi of seeking to foment "war between Iraqi Muslims".