In his speech to hundreds of thousands of Hezbollah supporters, Nasrallah said that his organisation "is siding with the resistance in Iraq".
"The Iraqis, Shia and Sunni, who took part in the political process wanted to give it a chance," he said.
"But now that the real American goal in Iraq has been exposed the Iraqi government is put to a test."
The address by Nasrallah, aired over a video link to supporters, was part of celebrations to mark eight years since Israeli forces withdrew from southern Lebanon.
It is the first time Nasrallah has issued a challenge to the Iraqi government to take a stand against the US military presence in Iraq.
"The Americans allowed the elections and the formation of parliament and a government so that they get an Iraqi legitimisation of the occupation," he said, referring to a reported Iraqi-American agreement that would allow the US to have a permanent presence in Iraq.
In downtown Beirut, security officials said at least nine people were injured after Nasrallah's speech.
Supporters of the ruling coalition and opposition loyalists traded insults before shooting at each other late on Monday.
The shooting took place in the Corniche Mazraa area, an avenue separating Shia and Sunni areas of Beirut.
The clashes suggest that while Lebanon's leaders have worked out an accord, their respective supporters may be more reluctant to observe the political détente.
Lebanese security officials say a soldier was killed during a gunfight between Hezbollah supporters and pro-government loyalists on Tuesday.
Hussein Mohammed Janadin was caught in the crossfire in the clash at a military post in the village of Aramoun, south of Beirut, officials said.
Two other men were injured when Hezbollah supporters beat them with sticks, the officials said.
The latest clashes come after Michel Sleiman, the former chief of the Lebanese army, was elected Lebanon's new president.
Sleiman was elected in a parliamentary session on Sunday after Sheikh Hamad bin Khalifa Al-Thani, the emir of Qatar, helped broker a deal to end a dispute between rival political factions in Lebanon.
In his speech to Hezbollah supporters, Nasrallah welcomed Sleiman's election as president, calling it a new chapter for Lebanon.
"The election of Michel Sleiman brings hope to the Lebanese of a new era and a new beginning," he said.
"His inaugural speech expressed the spirit of consensus that he promised to act upon in the upcoming period ... And this is what Lebanon needs."
Amal Saad-Ghorayeb, a political analyst and expert on Hezbollah, told Al Jazeera that Nasrallah's speech was "quite historic and strategic".
"On the one hand, many people expected him to be more apologetic about the events [clashes between pro- and anti-government fighters] of the last two weeks, but he was quite assertive," she said.
"For example, he talked not only about a defence strategy but a liberation strategy for Lebanon, in relation to the Shebaa Farms region [which is still occupied by Israeli forces]."
Zeina Khodr, Al Jazeera's correspondent in southern Beirut, said Nasrallah praised the resistance against Israel in the early part of his speech.
"He was giving credit to the resistance that actually led to the Israeli withdrawal from Southern Lebanon, even giving examples from Palestine and Iraq," she said.
But Khodr also said that he had sought to downplay suggestions that recent clashes in Beirut between pro- and anti-government forces were a sign of a growing sectarian divide in Lebanon.
"He said that in every country, there is disagreement - those who support resistance and those who do not," she said.
"He is trying to show that the battle and the power struggle in Beirut was not a Sunni-Shia clash - it was simply a power struggle between those who support the resistance and the other, who do not believe in the resistance's weapons."
Appeal for reconciliation
Nasrallah's speech came after Lebanon's 127-member parliament voted overwhelmingly in favour of Sleiman becoming president on Sunday.
In his inaugural speech, Sleiman appealed to Lebanese political factions to work together to avoid internal strife.
Nasrallah said on Monday that Hezbollah did not want to take over the country or undermine political institutions.
|Sleiman was elected Lebanon's president after|
a deal was brokered in Qatar [AFP]
"I am speaking as Hezbollah - we don't want power or authority, we don't want to rule Lebanon, we do not want to impose our thoughts on the Lebanese," he said.
"They call us the party of wilayat al-faqih," he said, referring to accusations that Hezbollah wants to impose an Iranian-style regime in Lebanon.
"The Lebanon wilayat al-faqih means the Lebanon of diversity and pluralism ... we should preserve it as such."
Lamis Andoni, Al Jazeera's Middle East analyst, said Nasrallah's speech, in regard to Lebanese politics, was conciliatory in tone.
"Nasrallah is aiming to recast himself as a nationalist leader and distance himself from the sectarian overtones that have marred the recent fighting," she said.
"His commitment to a pluralistic Lebanon reflects his keen awareness that recent confrontations have fuelled sectarian fears and widened the divide.
"However, Nasrallah did not compromise on rejecting any attempt, even if carried out by the Lebanese army, to disarm Hezbollah."