Monday's attacks on Islam's spiritual home came as Muslims prepare for the feast this week marking the end of the holy fasting month Ramadan.
Ziyaad Patel, 36, spoke exclusively to SBS News from Medina, where he had traveled from South Africa for the last days of Ramadan.
He was inside the mosque when a suicide bomber blew himself up in the car park outside, killing four people.
"After the prayer had concluded, there was a massive loud explosion and I was sitting on the marble floor," he told SBS News.
"When the explosion went off we could feel the vibrations coming from beneath the floor.
"Many people had thought perhaps it was a cannon, to say to us that tomorrow is Eid, that the crescent moon has been sighted.
"If this explosion happened on any other day there would've been a lot of a chaos, a lot of chaos."
Patel told SBS News the attack had left the country and the Islamic world in shock.
"This is the month of mercy," he said.
"This is the city of mercy. And coming here and seeing such disrespect, and coming here and doing such vile acts, such ugly acts right here in the place of mercy, right in the presence of the Prophet Mohammed; this is something which nobody will tolerate and nobody will accept."
There were no claims of responsibility, but the Islamic State group had urged its supporters to carry out attacks during the holy month and has claimed or been blamed for a wave of Ramadan shootings and bombings including in Orlando, Bangladesh, Istanbul and Baghdad.
The suicide bombing in Medina came during sunset prayers at the Prophet's Mosque - where Islam's Prophet Mohammed is buried and which attracts millions of pilgrims each year.
The Saudi interior ministry said in a statement that security forces became suspicious of a man who was 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 security personnel," the statement said, adding that five others were injured.
The targeting of Medina caused widespread outrage.
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 attacks "prove that those renegades... have violated everything that is sacred."
The head of Saudi Arabia's Shura Council, the kingdom's main advisory body, said the attack was "unprecedented".
"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.
Iran, the main Shiite power, also condemned the bombings and called for Muslim unity against extremists.
'No more red lines'
"There are no more red lines left for terrorists to cross. Sunnis, Shiites will both remain victims unless we stand united as one," Iranian Foreign Minister Mohammad Javad Zarif said on Twitter.
At the same time as the Medina attack, across the country in the Shiite-populated Gulf city of Qatif another suicide bombing took place near a Shiite mosque, residents and the interior ministry said.
The ministry said "the body parts of three people were found" at the site but had not yet been identified.
One witness told AFP there were two explosions near the mosque while another said he could see the torn apart body of what appeared to have been a suicide bomber.
The first attack on Monday had taken place in the western Saudi city of Jeddah, where two security officers were wounded when a suicide bomber blew himself near the US consulate in the early hours.
The interior ministry said the attack was carried out by a Pakistani resident identified as Abdullah Qalzar Khan, a 35-year-old private driver who had been living with his wife and her parents in Jeddah for 12 years.
Interior ministry spokesman General Mansour al-Turki told state news channel Al-Ekhbaria that the bomber was closer to a mosque in the area than to the consulate.
The ministry said in the statement published on the official SPA news agency that the bomber's explosive belt had "partially" exploded.
The US embassy in Riyadh reported no casualties among consulate staff. The attack coincided with the US July 4 Independence Day holiday.
Since late 2014 a series of bombings and shootings claimed by IS 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 many Shiites in the Sunni-dominated Gulf state.
IS 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 Shiites to be heretics.
IS spokesman Abu Mohamed al-Adnani had in late May urged the group's supporters to carry out attacks during Ramadan.
