In an address on state television, Abadi said only the army and police would be in the city. He called on people in Mosul to be in solidarity with the security forces.
It was reported earlier that the Iraqi army was gradually advancing toward Mosul, which officials in Baghdad vowed to liberate by the end of the year.
Peshmerga forces reportedly are deployed on the Khazir front -- 40 kilometers (25 miles) east of Mosul -- and U.S. artillery units also began striking Daesh positions on different fronts.
The primary targets of the offensive are the Hamdaniya and Karakus districts.
A peshmerga commander told Anadolu Agency that approximately 15,000 fighters would participate in the offensive on several fronts in eastern and southeastern Mosul.
Col. Seyit Khajar from the Khazir front said as many as 18,000 fighters could fight Daesh.
In Washington, U.S. Secretary of Defense Ash Carter described the launch as a "decisive moment" to completely defeat Daesh and said Washington has confidence in its Iraqi partners to free Mosul and the rest of Iraq from the terror group.
"The United States and the rest of the international coalition stand ready to support Iraqi Security Forces, Peshmerga fighters and the people of Iraq in the difficult fight ahead," Carter said in a statement.
In mid-2014, Daesh captured the northern city of Mosul and overran vast swathes of territory in northern and western Iraq.
Recent months have seen the army, backed by a 60-nation air coalition led by the U.S., retake a large portion of the territory. Nevertheless, the terrorist group remains in control of several parts of the country, including Mosul.