At least 14 people have been killed in a spate of attacks and fighting in northern Iraq, police say.
Friday's violence hit a number of northern Iraqi towns and cities
Ten people, including three policemen, were killed when a suicide bomber drove a truck with explosives on to a sports field in Hadhar, Nineveh province.
In Mosul, some 90km (55 miles) north of Hadhar, four policemen died as a fresh eruption of violence hit the city.
Several insurgents are also believed to have died in fighting there, shattering the fragile calm of recent months.
The Hadhar sports field attack happened as a group of people were playing football and basketball. At least seven people were injured.
Earlier, there was at least one car bombing and subsequent fierce fighting with insurgents in the northern city of Mosul, reports say.
A colonel was among the four police killed, with four people reported wounded. Several insurgents were also reported killed, with an unconfirmed police source saying eight had died in one exchange of fire.
A curfew has been imposed in Mosul, to add to those already in force in Iraq's two other major cities, Baghdad and Basra.
The BBC's Jane Peel in Baghdad says the sudden eruption of violence in Mosul will come as a surprise, as since the end of last year the city - once a hotspot for violence - has been largely calm.
In fact, the United States military recently announced it was withdrawing some 5,000 of its troops from Mosul to re-deploy them in the capital, Baghdad, which authorities fear they are on the brink of losing to insurgent control.
In other developments:
- In Baquba, just north of Baghdad, a pregnant woman and her husband were killed by a roadside bomb as they sped to hospital in the early hours of Friday to deliver the child
- Two marines were killed in separate incidents in western al-Anbar province, the US military said late on Thursday
- Three Iraqi civilians were killed and nine wounded on Thursday when an insurgents' mortar apparently aimed at US troops went astray in Obeidi, close to the Syrian border
- Gunmen shot and killed four members of a Shia family in Dujail, north of Baghdad, police told the Associated Press news agency
Meanwhile, large crowds of Shia Iraqis were converging on Baghdad for a rally backing Lebanon's Hezbollah group.
Reports from the scene say youths in white shrouds have gathered in Sadr City, a stronghold of Shia cleric Moqtada Sadr's Mehdi Army.
Waving yellow Hezbollah flags, and holding aloft pictures of the group's leader, Sheikh Hassan Nasrallah, they chanted: "Oh Allah, Oh Allah, make Hassan Nasrallah triumph!"
Tensions between the US military and supporters of Mr Sadr were high, following the deaths of two of the cleric's supporters on Thursday. The men were shot by US troops as they drove to Baghdad.
US military officials said troops in a watchtower shot at the men's vehicle only after they were shot at themselves.