All 25 members of Iraq's Governing Council signed the new interim constitution on Monday in a ceremony in Baghdad.
The constitution has originally been scheduled to be signed on Wednesday but was postponed following Tuesday's attacks on Shiite holy sites in Karbala and Baghdad.
The interim constitution is expected to take effect on July 1 after the US-led coalition authorities hand over sovereignty to a caretaker Iraqi government on June 30.
US President George W. Bush on Monday hailed the signing of Iraq's new interim constitution, calling it "a historic milestone" for the Iraqi people.
"The adoption of this law marks a historic milestone in the Iraqi people's long journey from tyranny and violence to liberty and peace. While difficult work remains to establish democracy in Iraq, today's signing is a critical step in that direction," Bush said in a statement issued by the White House.
"This document is an important step toward the establishment of a sovereign government on June 30th," Bush said, adding that he congratulated the Iraqi Governing Council and the Iraqi people "on completing the Transitional Administrative Law for Iraq."
Bush said that the interim constitution "lays the foundation for democratic elections and for a new constitution, which will be written by an elected Iraqi assembly and approved by the Iraqi people."
British Foreign Secretary Jack Straw on Monday welcomed the signing by the Iraqi Governing Council (IGC) of an interim constitution, describing that it as a "significant achievement."
"I would like to welcome the signing of Iraq's Transitional Law. This is a significant achievement and I want to pay tribute to all parties to this agreement, for their patience and for the spirit of consensus in which they have negotiated it," Straw told a joint news conference with visiting Palestinian Prime Minister Ahmed Qurei.
"This is the clearest demonstration to date of the strong desire to build a free, stable and democratic society in Iraq, despite the terrorist violence which seeks to deny the people of Iraq the future they want," Straw told reporters.
"The transitional law sets out the path Iraq will take to democracy. The people of Iraq will continue to have our full support as they work for their freedom," he added.
Russia welcomed the signing of the Iraqi interim constitution Monday saying it will help promote the country's sovereignty.
"We believe that the law on state government will promote the speedy recovery of Iraq's sovereignty and its position in the international arena," Russia's Foreign Ministry said in a statement.
"Iraq has developed the legal foundation required for preserving a single Iraqi state system, for the process of rebuilding the country on a democratic foundation and for resolving social, economic and other problems," the statement said.
The statement also expressed Russia's determination to further develop a "mutually beneficial" cooperation with Iraq.
Iraq's history of interim constitutions
In 1925 Iraq had a permanent constitution which was endowed by the monarch. It continued to be valid until 1958 when it was canceled as a result of a coup that made the country a republic.
The revolution established an interim constitution. Ever since, Iraq has been ruled with interim constitutions. Following are the key points of some major interim constitution versions:
July 1958: Iraq is an inalienable part of the Arab Nation. The Republic of Iraq is a sovereign state, independent and free. The people are the source of authority. There shall be no differentiation in the rights of Iraqis before the law, whatever differences may exist in language, race or creed. Islam is the state religion. The city of Baghdad is the capital of Iraq.
May 1964: The Republic of Iraq is based on Arab tradition and Islamic doctrines. Its basic objective is the realization of one Arab State. Islam is the state religion. Public and private ownership shall be safeguarded.
September 1968: Iraq is a "People's Democratic Republic." "Islam is the state religion." The political economy of the state is founded on socialism. The state protects private ownership. Thepost of prime minister is canceled. The President of Iraq is the head of state, the general commander of the armed forces and the chairman of the Council of Command of the Revolution (RCC) -- the highest authority in the country.
July 1970: The state should "develop socialism on the basis of science and revolution."
September 1995: The President of Iraq should be directly elected instead of being selected through nomination. Election comprises three stages: nomination of the RCC, adoption of the parliament and referendum. The president holds a tenure of seven years and can be re-elected.