The exchange, brokered by Turkey and Qatar, came days after Mr. Assad warned on Sunday that he would not abandon the fight against armed adversaries pressing on the approaches to the Syrian capital, Damascus, and brushed aside calls for him to quit.
The swap started in more than one location near Damascus after months of diplomatic efforts that also involved the Humanitarian Relief Foundation, an Islamist-leaning aid organization based in Istanbul and widely 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.
He added, “Efforts under the Turkish and Qatari mediation continue, while the exchange started at several locations where prisoners were kept.”
Mr. Yildirim told Reuters that, after their release, the 48 Iranians were escorted to Damascus by Iranian and Syrian officials.
The aid group has set up an operation center in Damascus to unite 2,130 prisoners, including 73 women, at one base while another aid team remained in Douma, near the Syrian capital, to oversee the return of the 48 Iranians.
The Syrian opposition has claimed that the Iranians are members of Iran’s Islamic 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 aid group’s 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.”
The aid group gained international attention in 2010 for organizing a flotilla of boats heading to Gaza, ostensibly with relief supplies, that prompted a deadly Israeli commando raid in which nine Turks died. At the time of the raid, the group was reported to have extensive connections with Turkey’s political elite. The episode began an unraveling of Turkey’s once close ties with Israel.
In recent months, the aid group has also been part of negotiations to free smaller numbers of prisoners, including two Turkish journalists held in Syria, Reuters reported. The organization has been active since the early 1990s in charitably works in Middle East and Africa, focusing most recently on Gaza.
Since the start of the uprising against Mr. Assad, the organization has also cast itself as a leading private charitable organization in Syria, delivering food and other basic supplies and pursuing what it calls “humanitarian diplomacy” to help free captive civilians.
While the numbers involved in Wednesday’s exchange seemed dramatic, some rebel commanders said more modest prisoner exchanges have become a feature of the conflict.
The leader of a rebel fighting group in the central city of Hama, reached via Skype, said pro-government militia had captured his uncle and two other relatives in a village in the northern Idlib province more than a month ago.
“The only way to release them is capturing hostages,” the commander said, adding that negotiations were under way to win the release of his relatives in return for 12 captives held by the rebels. Two months ago, the commander said, nine members of the pro-government shabeeha militia were exchanged for five captured rebels. 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.
Sebnem Arsu reported from Istanbul and Alan Cowell from London. Hwaida Saad contributed reporting from Beirut, Lebanon.
