George Bush, the US president, said on Thursday that al-Zarqawi's death in a US airstrike on a village north of the Iraqi capital, Baghdad, was a "severe blow" to the al-Qaeda network.
"It's a victory in the global war on terror and an opportunity for Iraq's new government to turn the tide of this struggle," Bush told reporters at the White House.
Tony Blair, the UK prime minister, also described news of the Jordan-born al-Zarqawi's death as "very good news", while Kofi Annan, the United Nations secretary-general, said he was relieved at the death of such a "heinous and dangerous" man.
Leaders in Japan, Australia, Afghanistan and Pakistan also welcomed the news.
However, both Bush and Blair warned that al-Zarqawi's death would not put an end to the bloody insurgency currently raging in Iraq.
"We know they will continue to kill, we know there are many, many obstacles to overcome," Blair said. "But they also know that our determination to defeat them is total."
There was also a mixed reaction from relatives of those purportedly killed by al-Zarqawi's group in Iraq.
The family of Kenneth Bigley, a UK contractor kidnapped and later beheaded - allegedly by al-Zarqawi himself - described al-Zarqawi as a monster who had killed "a multitude of innocent people".
"I'm glad he's off the face of the earth, not just for my brother but for all the people he has killed"
Brother of UK contractor killed in Iraq
"I'm glad he's off the face of the earth, not just for my brother but for all the people he has killed," his brother, Stan, told the British media.
But the father of American Nick Berg, who was also killed by al-Zarqawi's group, said he felt no satisfaction at the death of the man who had reportedly killed his son.
"Revenge is something that I do not follow [and] I do not wish for against anybody," he told the CNN television network.
Al-Zarqawi's death was announced by Nuri al-Maliki, the Iraqi prime minister, at a press conference in Baghdad on Thursday morning.
Seven aides died along with
al-Zarqawi in the US air strike
He said al-Zarqawi was killed along with seven aides on Wednesday evening in a US air strike on a house 50km northeast of Baghdad, in the province of Diyala, just east of the provincial capital, Baquba.
American F-16 fighter jets dropped two 500lb bombs on the site.
Al-Zarqawi's identity was confirmed by fingerprints and pictures of his body displayed at a press conference by US military spokesman Major General William Caldwell.
Al-Maliki said the air strike was the result of intelligence reports provided to Iraqi security forces by residents in the area and US forces acted on the information.
Following the air strike, 17 raids were conducted in the Baghdad area in a bid to find associates of al-Zarqawi. The raids provided a "treasure trove" of information, US officials said.
Al-Qaeda in Iraq, the organisation headed by al-Zarqawi, has vowed to fight on in an internet statement, reports said.
Meanwhile violence in Baghdad continued on Thursday, with at least 39 people dying in three bomb attacks across the city.