The Muslims appeared before Alexandria in March 641. Alexandria was heavily fortified. There were walls behind walls, and forts within forts. The Byzantine force within the city numbered 50,000 while the strength of the invading Muslim force was 1,000 only. There was no dearth of provisions and food supply in the city. The city had direct access to the sea, and through the sea route help from Constantinople in men and material could come any time.

As Amr surveyed the military situation, he felt that Alexandria would be a hard nut to crack. The Byzantines had high stakes in Alexandria, and they were determined to offer stiff resistance to the Muslims Amr, however, felt that in spite of the heavy odds the Muslims would be able to conquer the city. The Muslims accordingly decided to lay siege to the city. The Byzantines mounted catapults on the walls of the city, and these engines pounded the Muslims with boulders. This caused considerable damage to the Muslims and Amr ordered his men back from the advance position so that they might be beyond the range of these missiles.

A see-saw war followed. When the Muslims tried to go close to the city they were pounded with missiles. When the Byzantines sallied from the fort, they were invariably beaten back by the Muslims.

Heraclius the Byzantine emperor collected a large reinforcement at Constantinople. He intended to march at the head of this reinforcement personally to Alexandria. Before he could finalise the arrangements he died. The reinforcement mustered at Constantinople dispersed, and no help came to Alexandria.

When the Muslims came to know that the Byzantine emperor had died and that no reinforcement was likely to come to Alexandria they intensified their attacks. In one of the assaults the Muslims got into one of the towers. On the Byzantine counter attack the Muslims withdrew. As the Byzantines closed the outer gate four Muslims were trapped inside. These four Muslims descended to an underground chamber. Because of the narrowness of the passage it was not possible for the Byzantines to descend to the chamber to capture these Muslims alive. Left to themselves these Muslims would have been starved to death within a few days. Among these four trapped Muslims were Amr b. Al-Aas the Commanderin-Chief of the Muslim force; Masalma bin Mukhallad a young stalwart, and two others. The Byzantines were not aware of the identity of these four Muslims. They took them to be ordinary soldiers of no particular significance.

In a playful mood the Byzantines asked these trapped Muslims to surrender for if they did not do so they would automatically die in the underground cellar within a few days. The Muslims refused to surrender. Thereupon the Byzantines said that they could be exchanged with Byzantine prisoners in the Muslim camp. This was also not agreed to by the trapped Muslims. Thereupon in a chivalrous mood the Byzantines said, "Let us have a duel, one man out of you and one man from us. If your man kills our man, all of you can depart. If your man is killed the rest of you will be our captives". To this the Muslims agreed.

Amr wanted to offer himself for the duel, but Masalma a young man of great sinews prevailed upon him that he should let Masalma fight the duel Amr ultimately agreed. The Byzantines gave a solemn undertaking in the terms of the agreement arrived at and the trapped Muslims came out of the cellar into the chamber where the duel was to be held.

The Byzantine champion stepped forward and he was met by Masalma from the Muslim side. The contest was hard and stiff, and it appeared as if the Byzantine champion would score. But ultimately Masalma scored and the Byzantine champion was killed. The Byzantines kept their word. After the duel was over they opened the gate of the tower and let the Muslims go in peace. Little did they know that these four included the Commander-in-Chief of the Muslim force.

The state of stalemate continued. The Muslims intensified their attacks but there was no slackening of the Byzantine resistance. The siege dragged on for six months, and in Madina Umar got impatient. In a letter addressed to Amr the Caliph expressed his concern at the inordinate delay in the conquest of Egypt. Umar wrote:

"When you get this letter address the people and urge them to fight. Launch the attack in the early afternoon of a Friday for that is the hour of God's blessing."

Amr bin Al-Aas assembled his men, and read to them the letter of Umar. Fiery speeches were held to inspire the Muslims to violent action. It was decided that after the ensuring Friday prayers an all-out assault would be launched on the enemy. Ubada was chosen to carry the standard and launch the assault.

The following Friday after the noon prayers, the Muslims marched to the battle-field with the coffins tied on their heads. They moved forward with the fury of a torrent, and all resistance was swept aside. Before the sun set the city was carried by the Muslims by storm. Over 20,000 Byzantines were killed or taken captive. The rest of the Byzantine army found safety in flight to Constantinople through ships that stood anchored in the port. Some wealthy traders also left.

On behalf of the Egyptians, Maqauqas sued for peace, and peace was allowed. In his report to the Caliph, Amr reported:

"We have conquered Alexandria. In this city there are 4,000 palaces, 400 places of entertainment, and untold wealth."

The Muslim soldiers were keen to collect the war spoils and distribule them among themselves. Maqauqas pleaded that in pursuance of the terms of the treaty those Egyptians who had chosen to remain in the city could not be deprived of their belongings or property. Most of the Muslim soldiers were of the view that as Alexandria had been taken by sword the Muslims had the right to the spoils of war. The matter was referred to Umar, and he decided that while the Muslims could appropriate all the property and assets of the former Government, the private property should not be touched if the owners were there.

With the fall of Alexandria the Muslims were the masters of Egypt.

After the fall of Alexandria, Amr bin Al-Aas deputed a fast rider Muawiyah bin Khudaij to carry the news of the victory of the Muslims to Umar at Madina. When Muawiyah reached Madina it was noon. Muawiyah thought that Umar would be resting at the time and it was inadvisable to disturb him. He accordingly went to the Prophet's mosque to await the arrival of the Caliph there to lead the afternoon prayers. A slave girl of Umar who was passing that way happened to see the traveller. Her curiosity having been awakened she enquired from the traveller from where he had come and he said that he was coming from Alexandria. The slave girl knew how Umar had been anxiously awaiting news from Alexandria. She accordingly rushed home and told Umar that a man had come from Alexandria. Umar asked the slave girl to go to the mosque to fetch the messenger from Alexandria.

As Muawiyah presented himself, Umar anxiously enquired what news he had brought. Muawiyah said that he carried good news and that God in His mercy had given victory to the Muslims. Umar then enquired from Muawiyah why did he not come straight to him. Muawiyiah said that he thought the Caliph would be resting and it was inadvisable to disturb him at that hour of the day. Thereupon Umar said, "I am sorry that you have such a poor opinion of me. Who would bear the burden of the Caliphate, if I were to sleep during the days?"