BAQUBA (AFP) - Iraqi forces backed by US troops launched a major assault on Tuesday against fighters in the province of Diyala, an Al-Qaeda stronghold and one of the most dangerous places in the country. "The operation began in Diyala early this morning and we have begun raids in some neighbourhoods of the city of Baquba," said Ragib al-Omeiri, chief of the operations bureau in Baquba. "Iraqi police and the Iraqi army are working together with the US army," he told AFP. In sweeps that aim to clear out the Sunni Al-Qaeda bastion in the northeast of the country, Iraqi troops netted 20 suspected insurgents, said Defence Ministry spokesman Mohammed al-Asskri. US army spokesman Major John Hall said the security manoeuvres had been exclusively planned and executed by Iraqi forces, a signal of their increasing field competence as American troops took on a secondary role. Meanwhile, under a mass of green and black flags, thousands of pilgrims walked through Baghdad's heavily-policed streets for a major religious ceremony on Tuesday, a day after women suicide bombers killed at least 25 Iraqis. Officials said more than a million people crowded the holy shrine of Imam Mussa Kadhim in Kadhimiyah, in northern Baghdad, where an extra 5,000 police and soldiers have been deployed as part of stepped up security, which also included a traffic curfew and additional checkpoints. Volunteers handed out free water and food to passing pilgrims, many of whom had walked all day in defiance of the threat of violence in the Iraqi capital of six million residents. "Despite the explosions that happened, I can see that people are united and determined to complete the visit to the Kadhimiyah shrine," said Alaa Abdul Hussein, as one of his sons carried a flag that read: "Peace be with you." Tuesday's mass of pilgrims was swollen by crowds of women, each apparently indistinguishable from the next in their all-enveloping black chadors, watched as they passed by armed security forces. No incidents of fresh violence were reported during Tuesday's mass pilgrimage to mourn the death 12 centuries ago of Shia Imam Mussa Kadhim. He is believed to have been buried in the Kadhimiyah mosque after being poisoned in Baghdad in the late eighth century by agents of the then-ruling Sunni caliph, Harun al-Rashid. "People come here for only one reason, to give their condolences to Imam Kadhim," said Hussein Karim, a teacher from Baghdad who tried to give passing pilgrims some respite from the heat by spraying them with water.