* Bombs hit mainly Shi'ite areas
* Fighting north, west of Baghdad has taken heavy civilian toll
* Army, with Shi'ite militias, battling Sunni insurgents (Adds top Shi'ite cleric's meeting with UN envoy)
By Raheem Salman
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.
No person or group immediately claimed responsibility for the attacks.
Islamic State, an al Qaeda offshoot 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 experienced few attacks compared with other areas targeted by the Islamic State's offensive last month, though bombs still hit the capital on a fairly regular basis.
At least 5,576 Iraqi civilians have been killed since January, when Sunni insurgents led overran the city of Falluja in the western province of Anbar, the United Nations said on 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, which swept across northern Iraq last month, advanced to within 70 km (45 miles) of Baghdad. The Iraqi army and allied Shi'ite militia have been trying since then to regain the territory.
SISTANI, U.N. ENVOY MEET
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 on Saturday that government forces were fully in control of the base.
The fighting has exacerbated a political crisis in Baghdad, where Maliki is trying to form a new government in the face of opposition from Sunnis, Kurds and some fellow Shi'ite politicians, three months after Iraq's parliamentary election.
Iraq's Shi'ite clergy as well as Western powers have pressed politicians to overcome their deadlock and agree on a unity government to tackle the insurgency and prevent Iraq from splitting down ethnic and sectarian lines.
The country's top Shi'ite cleric, Grand Ayatollah Ali al-Sistani, met Nickolay Mladenov, the U.N. special envoy to Iraq, on Saturday.
Sistani, an ascetic 83-year-old of almost mythological stature to his followers, has taken his most active role in politics in a decade during the current crisis.
He has demanded politicians choose a new government without delay and urged fighters to respect the rights of all Iraqis, regardless of sect or politics, after an escalation in apparent sectarian killings in recent weeks.
"We are in full agreement with His Eminence (Sistani) on the need to expedite the government formation process and form a new government that is acceptable to all communities," said Mladenov after the meeting.
(Reporting by Raheem Salman, Dominic Evans and Maggie Fick; Editing by Louise Ireland and Sophie Hares)