By Alastair Macdonald and Ross ColvinBAGHDAD, Nov 24 (Reuters) - Gunmen attacked a Sunni Arab neighbourhood of Baghdad and burned mosques on Friday in apparent retaliation for the bloodiest bombing in more than three years of war that killed 202 in a Shi'ite area.Two suicide bombs ripped through a Shi'ite market in northern Iraq killing 22 people earlier on Friday and mortars crashed on rival Baghdad neighbourhoods, ramping up sectarian tension that threatens to push Iraq into all-out civil war.As political leaders on all sides pleaded for restraint and imposed a curfew on the capital, gunmen stormed a Sunni enclave in a largely Shi'ite area, burning four mosques and homes, an Interior Ministry official said.The official said the number of casualties was not known, but a resident of Hurriya district, Imad al-Din al-Hashemi, said at least 18 people had been killed and 24 wounded."They attacked four mosques with rocket-propelled grenades and machinegun fire. The attacks began at midday," said Hashemi, a university academic, who was helping evacuate people.