Riyadh - Outrage spread on Tuesday after a deadly suicide bombing at Islam's second-holiest site in the Saudi city of Medina, one of three attacks in the kingdom on a single day.Religious and political leaders across the Middle East denounced the attack near the Prophet's Mosque that left four dead and came as Muslims prepare for the feast this week marking the end of the holy month Ramadan.There were no claims of responsibility for Monday's bombings in Medina, Jeddah and the eastern city of Qatif, but the Islamic State group (ISIS) had urged its supporters to carry out attacks during Ramadan. The jihadist group has claimed or been blamed for a wave of shootings and bombings during the holy month this year, including in Orlando, Bangladesh, Istanbul and Baghdad.The suicide bombing in Medina came during sunset prayers at the mosque - where Islam's Prophet Mohammed is buried and which attracts millions of pilgrims each year.The Saudi interior ministry said officers became suspicious of a man heading for the Prophet's Mosque through a parking lot."As they tried to stop him, he blew himself up with an explosive belt causing his death and the death of four police," the statement said, adding that five others were wounded.The targeting of Medina caused widespread outrage, both in Saudi Arabia and across the Muslim world.The head of Saudi Arabia's Shura Council, the kingdom's main government advisory body, called it an "unprecedented" incident.'No more red lines' "This crime, which causes goosebumps, could not have been perpetrated by someone who had an atom of belief in his heart," Abdullah al-Sheikh said.Cairo-based Al-Azhar, the highest authority in Sunni Islam, condemned the attacks and stressed "the sanctity of the houses of God, especially the Prophet's Mosque."Saudi Arabia's supreme council of clerics said the blasts "prove that those renegades... have violated everything that is sacred."The attack drew condemnation across Islam's religious divide, with Shiite power Iran calling for Muslim unity after the attacks in its Sunni-dominated regional rival."There are no more red lines left for terrorists to cross. Sunnis, Shi'ites will both remain victims unless we stand united as one," Iranian Foreign Minister Mohammad Javad Zarif said on Twitter.Lebanon-based Shiite militant group Hezbollah - which Saudi Arabia accuses of supporting "terrorist acts" across the region - also denounced the Medina attack as "a new sign of the terrorists' contempt for all that Muslims consider sacred."The governments of Turkey and Lebanon joined in the condemnation, while Iraq said the attacks amounted to "heinous crimes".Middle East expert Madhawi al-Rasheed said the attack in Medina appeared aimed at humiliating the Saudi government, the guardian of Islam's holiest sites."It's an attempt to actually embarrass the Saudi government because it boasts of protecting the pilgrims and the holy places," said Rasheed, a visiting professor at the National University of Singapore's Middle East Institute.There also seemed to be an "organised effort by the perpetrators to coordinate their work," potentially signalling a worsening security situation in Saudi, she said. ISIS urged Ramadan attacks At the same time as the Medina attack, another suicide bombing occurred near a Shiite mosque across the country in the Shiite-populated Gulf city of Qatif.The Saudi interior ministry said "the body parts of three people were found" at the site but had not yet been identified.Nasima al-Sada, a Qatif resident, called the attackers crazy and said: "I don't know where they get this idea."Monday's first attack happened in the western Saudi city of Jeddah, where two police officers were wounded in a suicide bombing near the US consulate in the early hours.The interior ministry said a Pakistani resident, Abdullah Qalzar Khan, 35, carried out the Jeddah attack. He had been living with his wife and her parents in the city for 12 years.The US embassy in Riyadh reported no casualties among consulate staff during the attack, which coincided with the US July 4 Independence Day holiday.Since late 2014 a series of bombings and shootings claimed by ISIS in Saudi Arabia has targeted minority Shiites as well as members of the security forces, killing dozens of people.Most of the attacks have been staged in Eastern Province, home to the majority of Shiites in the Sunni-dominated Gulf state.ISIS leader Abu Bakr al-Baghdadi has called for attacks on Saudi Arabia, which is taking part in the US-led coalition bombing the jihadists in Syria and Iraq. The group also considers Shi'ites to be heretics.ISIS spokesperson Abu Mohamed al-Adnani had in late May urged the group's supporters to carry out attacks during Ramadan.