* Bombs hit mainly Shi'ite areas
* Fighting north, west of Baghdad taken heavy civilian toll
* Army, with Shi'ite militias, battling Sunni insurgents (Adds sixth bombing, army comments)
BAGHDAD, July 19 (Reuters) - At least 27 people were killed in a wave of bombings in mostly Shi'ite Muslim areas of Baghdad on Saturday, police and medics said, in the deadliest day of attacks in the capital since a Sunni insurgency overran large parts of Iraq's north last month.
In the first explosion, a suicide car bomber drove into a police checkpoint, killing nine people including seven policemen and wounding 21 people in the Abu Dsheer district in the south of the capital, the sources said.
Four other car bombs killed a total of 19 people: one in the Bayaa district in southwestern Baghdad, one in the western district of Jihad and two in northern Baghdad's Kadhimiya, the site of a major Shi'ite shrine.
One person was killed when a bomb laid on the side of a road exploded in the mixed Sunni-Shi'ite district of Saydiya in southern Baghdad later in the day.
There were no immediate claims of responsibility for the attacks.
Islamic State, the militant group that seized large parts of northern Iraq last month, has claimed several suicide bombings in the capital. Its latest claim was for a bombing that killed three people on Thursday in the heart of the city.
Baghdad has seen few attacks compared to the violence in other areas hit by the Islamic State's offensive last month, though bombs still hit the capital on a fairly regular basis.
The civilian toll, mainly from fighting between government forces and the Sunni insurgents, this year has been huge.
At least 5,576 Iraqi civilians have been killed since January, when Sunni insurgents led by an al Qaeda offshoot now known as Islamic State, overran the city of Falluja in the western province of Anbar, the United Nations said Friday.
The U.N. said more than 1.2 million people have been displaced this year. More than 600,000 of them have fled their homes since early June.
The patchwork of Sunni insurgents led by Islamic State that swept across northern Iraq last month reached to within 70 km (45 miles) of Baghdad. The army and allied Shi'ite militia have been trying since then to regain the territory.
Militants fought off an army offensive to retake the northern city of Tikrit on Tuesday. The army was forced to pull back south of the city on the banks of the Tigris.
Intense fighting has raged for days northwest of Tikrit around a military base known as Camp Speicher, once one of the main U.S. headquarters. Islamic State wrote on an affiliated Twitter feed on Thursday it had shot down two helicopters during a battle around the base.
Prime Minister Nuri al-Maliki's military spokesman said Saturday government forces were fully in control of the base.
The fighting has exacerbated a political crisis in Baghdad, where Shi'ite caretaker Prime Minister Nuri al-Maliki is trying to form a government in the face of opposition from Sunnis, Kurds and some Shi'ites, three months after Iraq held a parliamentary election.
Iraq's Shi'ite clergy as well as Western powers have pressed politicians to overcome their deadlock and agree a new unity government to help tackle the insurgency and prevent Iraq from splitting down ethnic and sectarian lines. (Reporting by Raheem Salman, Dominic Evans and Maggie Fick; Editing by Louise Ireland and Sophie Hares)