* Attacker wore suit to blend in with mourners
* Sunni militancy revitalised by Syrian crisis
* Fourth suicide bombing in a week in Iraq
* Twenty two dead bring toll this week to at least 55 (Adds details on protests, context)
BAGHDAD, Jan 23 (Reuters) - A suicide bomber disguised as a mourner killed at least 22 people inside an Iraqi Shi'ite Muslim mosque on Wednesday when he set off his explosives in the middle of a crowded funeral.
The latest of four suicide attacks in a week came as Shi'ite Prime Minister Nuri al-Maliki faces mounting pressure from mass Sunni protests that threaten to return Iraq to the scale of sectarian violence that killed thousands in 2006-2007.
Dressed in a suit, the bomber mingled with mourners before detonating the blast at the Saif al Shuhada, Sword of the Martyrs, mosque in Tuz Khurmato city at a ceremony for a Shi'ite ethnic Turkman, police and witnesses said.
"I was sitting in the seats at the back when all of sudden I heard the sound of a huge explosion. Thank God I was behind because people in front of me saved me with their bodies," said Abbas Qadir Mohammed, 35, one of the wounded.
Panicked survivors packed the injured and dead wrapped in carpets into trucks and cars to rush them to hospital in the religiously and ethnically mixed city 170 km (105 miles) north of the capital Baghdad.
Rigot Mohammed, an Iraqi army spokesman, said at least 22 people were killed and more than 50 wounded in the blast.
No one claimed responsibility, but al Qaeda's local wing, Islamic State of Iraq, often targets Shi'ite Muslim pilgrims and sites to try to trigger widespread Sunni-Shi'ite confrontation in Iraq a year after the last American troops withdrew.
The Sunni militant group has pledged to win back ground lost during the war following the U.S.-led 2003 invasion and has been reinvigorated by Sunni Islamists fighting against President Bashar al-Assad in neighbouring Syria.
Maliki is struggling to calm weeks of street protests by Sunni Muslims while his delicately balanced government, a coalition of Sunnis, ethnic Kurds and the Shi'ite majority, is deadlocked in a crisis over power sharing.
The attack on Tuz Khurmato was the fourth suicide bombing in Iraq in a week. At least 55 people have been killed in the blasts, including a Sunni lawmaker visiting a town where protesters were gathering. Al Qaeda claimed responsibility for that bombing.
Sunni unrest and renewed violence in Iraq are compounding fears the war in neighbouring Syria - where Sunni rebels are battling to topple Assad, an ally of Shi'ite Iran - will upset Iraq's own fragile sectarian and ethnic mix.
The local al Qaeda affiliate's position has been boosted by Sunni Islamist fighters, cash and arms flowing to the war in Syria.
Sunni protests erupted in late December after authorities arrested the bodyguards of a Sunni finance minister on charges of terrorism. Sunni leaders said the arrests were part of a crackdown from the Shi'ite-led government on their sect.
Since the fall of Sunni strongman Saddam Hussein in 2003 and the rise of the Shi'ite majority to power through elections, many Iraqi Sunnis say they feel they have been marginalised over the decade since the U.S.-led invasion.
Protesters want reforms to an anti-terrorism law and an easing of a campaign against members of Saddam's former Baath party, two measures they say officials use unfairly to target the Sunni community.
Maliki has appointed a top Shi'ite figure to investigate those conditions and authorities have freed nearly 1,000 detainees in a gesture to appease protests. But demonstrators are calling more and more for the Shi'ite premier to step down. (Reporting by Mustafa Mohammed and Omar Mohammed in Kirkuk, and Aseel Kami in Baghdad; Writing by Patrick Markey; Editing by Louise Ireland)