"I announce victory from the heart of Mosul. I announce the collapse of the state of falsehood Daesh had declared in Mosul," al-Abadi said in remarks aired on Iraqi state TV.
Abadi said the victory against terrorism was achieved with the efforts of Iraqi security forces and the Iraqi people in general. He also thanked all countries who provided support in the fight against terror in terms of training, logistics and airstrikes.
"Our next mission is to achieve stability, rebuild infrastructure and destroy Daesh cells," Abadi said, adding this required the same unity which helped Iraq fight against Daesh.
He then raised an Iraqi flag as he said "Long live Iraq".
The U.S.-led coalition also confirmed on Monday the long-sought victory, saying that while small pockets of Daesh fighters and explosive devices might remain in the city, Iraqi forces "have Mosul now firmly under their control".
"Make no mistake; this victory alone does not eliminate ISIS [another acronym for Daesh] and there is still a tough fight ahead. But the loss of one of its twin capitals and a jewel of their so-called caliphate is a decisive blow," Coalition Commander Lt. Gen. Stephen J. Townsend said in a statement.
"Now it is time for all Iraqis to unite to ensure ISIS is defeated across the rest of Iraq and that the conditions that led to the rise of ISIS in Iraq are not allowed to return again," Townsend added.
The coalition said it would continue to assist Iraqi forces in fully ridding Iraq of Daesh's forces until "all Iraqis are liberated and security is restored".
U.S. Secretary of State Rex Tillerson also hailed the victory, calling it a "critical milestone" in the fight against Daesh .
Mosul was captured by Daesh along vast swathes of territory in northern and western Iraq in 2014.
The Iraqi army, backed by a U.S.-led air coalition, had since last October been battling to dislodge Daesh from Mosul, the terrorist group’s last stronghold in northern Iraq.