Convoys of Iraqi and Kurdish forces were moving towards the east of the city, as US-led coalition airstrikes sent plumes of smoke into the air amid the sound of heavy artillery rounds.
Some 2,000 Iraqi special forces are being supported by four brigades of the regular Iraqi Army, 15,000 Sunni militia, 15,000 Kurdish Peshmerga and a few thousand Shia militia.
They are taking the fight to an estimated 3,000 IS fighters in the city, with snipers sent in first to fight street-by-street.
According to Sky News, “The fear is that Mosul will be rigged with explosives. The other fear is chemical weapons. There is no doubt Islamic State has the capability and it’s possible the group has been saving the weapons for ‘D-Day’.”
Announcing the operation on Iraq’s Iraqiya TV channel, Prime Minister Haider al Abadi said: “The time of victory has come and operations to liberate Mosul have started.”
Addressing the residents of the city he added: “Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh (IS).”
Earlier the Iraqi army dropped thousands of leaflets over the besieged city warning more than a million civilians of the imminent military offensive.
Lieutenant General Stephen Townsend, commander of the US-led coalition, said the assault could last weeks – “possibly longer”.
“This may prove to be a long and tough battle, but the Iraqis have prepared for it and we will stand by them,” he said.
Most of the coalition’s support has come in the shape of airstrikes and training, but US, French and British special forces are now also on the ground to advise local forces in battle.
The push to retake the city is the biggest military operation in Iraq since American troops left in 2011.
Mosul was overrun by heavily armed Islamic State militants in 2014 in what was described as a “total collapse” of government security forces, causing thousands of families to flee to Iraq’s autonomous Kurdish region in the north.
IS leader Abu Bakr al Baghdadi visited the city to declare an Islamic caliphate which at one point covered nearly a third of Iraq and Syria.
Using Mosul as a base, the jihadi group swept further south through Iraq’s towns and cities, but government soldiers trained by US forces have since gained ground and Mosul is now the last city in the country held by IS.
As the assault got under way, Turkey’s President Recep Tayyip Erdogan also signalled his country’s involvement, saying: “Our brothers are there and our relatives are there. It is out of the question that we are not involved.”
According to UN estimates, up to one million people could be displaced from Mosul during the operation, exacerbating the humanitarian situation in the country.
It said families are at “extreme risk” of being caught in crossfire, tens of thousands may end up besieged or held as human shields and thousands could be forcibly expelled.
Safe escape routes out of the city for civilians caught up in the bloody conflict “do not exist”, Save the Children warned.
Families have been advised by local forces to stay inside and erect a white flag on their homes to stay safe.
There are more than 7,500 coalition military personnel deployed in Iraq, more than half of them from the US.