The rally coincided with a closed session of parliament during which Foreign Minister Mohammad Javad Zarif, who led Iran's negotiating team in the intense talks in Switzerland, appeared before lawmakers to explain the framework agreement.
The gathering of about 200 hard-liners took place in front of the parliament in the Iranian capital without permission by authorities, IRNA reported.
Protesters carried banners calling the agreement a "defeat" — despite the establishment's overwhelming backing of the deal, including by President Hassan Rouhani.
The deal, which is to be finalized by June 30, is to curb Iran's bomb-capable technology while giving Tehran quick access to bank accounts, oil markets and financial assets blocked by international sanctions.
The West has long suspected Iran's nuclear program is geared for weapons-making, a charge Tehran denies, insisting it's only for peaceful purposes like power generation and cancer treatment.
Since the agreement was announced last Thursday in Lausanne, Switzerland, Iranian hard-liners have also claimed it will provide international monitors from the UN nuclear watchdog agency — the International Atomic Energy Agency — access to Iranian military sites, though that has not been part of the framework.
Iran's deputy foreign minister, Hassan Qashqavi, said Tuesday's parliament session "useful" and urged lawmakers to support Zarif and his team of negotiators ahead of further talks in finalizing the agreement.
Another lawmaker, Behrouz Nemati said the "majority of lawmakers were satisfied" with Zarif's report.
