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The exchange, brokered by Turkey and Qatar, came days after Mr. Assad warned that he would not abandon the fight against rebels pressing on the approaches to Damascus and brushed aside calls for him to quit.

The prisoner swap started in more than one location near Damascus after months of diplomatic efforts that also involved the Humanitarian Relief Foundation, an Islamist humanitarian aid organization based in Istanbul known as I.H.H., according to Bulent Yildirim, the head of the foundation.

"Some of our friends are with the opposition groups and we are heading to the area where the prisoners are to be released," Mr. Yildirim told Turkey's semiofficial Anatolian news Agency.

"Efforts under the Turkish and Qatari mediation continue, while the exchange started at several locations where prisoners were kept."

The I.H.H. has set up an operation center in Damascus to unite 2,130 prisoners, including 73 women, at one base while another I.H.H. team remained in Douma, near the Syrian capital, to oversee the return of 48 Iranians.

The Syrian opposition has claimed that the Iranians are members of the Iran's Revolutionary Guards Corps, but Tehran has denied the assertion, saying the captives are Shiite civilian pilgrims. The Iranians were seized in August while traveling on a bus from Damascus International Airport to a Shiite shrine on the outskirts of the capital, Iran's Press TV said.

Opposition fighters had threatened to kill the Iranians unless Mr. Assad's forces halted military operations. But since then the fighting around Damascus has intensified.

Iran is Mr. Assad's main ally in a region where many Arab states and neighboring Turkey have turned against him, seeking his ouster. The Iranian captives offered the rebels holding them a source of pressure on the Syrian leader to push for the release of prisoners.

"We expect the swap to be completed in the next hour," Huseyin Oruc, a member of the I.H.H. executive board said in a telephone interview around midday.

"Among the 2,130 there are Syrians, four Turks, and a Palestinian, names of whom we will announce later in the day."

Diplomatic efforts to release more prisoners would continue, he added.

"It is the first time that the 'humanitarian diplomacy' we initiated succeeded in releasing such a large group of people at once," Mr. Oruc said."There are many more held captive and our efforts to free them will continue without delay."

I.H.H. became gained international attention in 2010 for organizing a flotilla of boats heading to Gaza that prompted a deadly Israeli commando raid in which nine Turks died. At the time of the raid, the I.H.H. was reported to have extensive connections with Turkey's political elite.

Syria's uprising began in March 2011 with peaceful demonstrations, but a brutal crackdown broadened into civil war with an estimated 60,000 people killed, according to United Nations estimates.

  In a speech on Sunday, Mr. Assad sidestepped demands for his departure and vowed to continue the battle against what the government calls terrorists.