The participants, who were wearing yellow vests with pictures of the leaders of the group, Maryam and Massoud Rajavi, chanted, "We want justice, we want peace, we want MEK off the list" and also "Iran, Rajavi, Rajavi, Iran."
The State Department is reviewing the status of the MKO, which it put on the list of terrorist organizations in 1999. The group, which has been described by former members as a Marxist/Islamist cult, is also designated as a terrorist organization by Iran.
The MKO, which was involved in a series of attacks in Iran in the 1980s, says it has renounced violence and is working for democracy in Iran. The group sided with Iraq during the bloody Iran-Iraq War in the 1980s, and more than 3,000 members are still based in Camp Ashraf in Iraq.
Today's event included speeches by several former U.S. officials and congressmen, including former Representative Patrick Kennedy (Rhode Island-Republican) who said in Persian, " I am Iranian, I am an Ashrafi," referring to the camp in Iraq. He also warned Iraqi Prime Minister Nuri al-Maliki that he will be put on trial in the International Criminal Court for attacking Camp Ashraf.
Organizers said the event was broadcast live for Camp Ashraf members. Several rally participants told RFE/RL they have family members in Ashraf.
One of the organizers, who identified herself as Shirin, told RFE/RL the removal of MKO from the list of foreign terrorist organizations would allow MKO members in Ashraf to move to other countries.
Shirin -- who said she had been jailed in Iran in 1981 for 2 1/2 years and tortured -- said the MKO's removal from the list would allow those who are "hesitant" because of this status to join the opposition group.
Rally participants also included a number of non-Iranian-Americans. Several of them told RFE/RL they had little or no knowledge about the MKO. "We've come for Iranian people," said one African-American.
When asked whether he and his female companion received money to attend the rally, the woman first said yes, but then retracted her statement at the man's behest.
Another MKO supporter said describing the group as a cult is a label by intelligence agencies, which he said do not understand the nature of the group. In response to a question about the whereabouts of MKO leader Massoud Rajavi, who hasn't been seen for years, he said his location should be kept secret for his personal safety.
Today's rally was the latest event in a series the MKO has held in recent months in Washington to push for its delisting. Western media have reported that former U.S. officials have received substantial financial contributions to speak at MKO events and express support for the delisting of the group.
Members and supporters of Iran's opposition Green Movement, on the other hand, have warned about the removal of the group from the U.S. terror list.
Tehran-based political analyst Nejat Bahrami told RFE/RL earlier this month the delisting of the MKO would make average Iranians frustrated with the United States.
"The messages [U.S. President Barack] Obama has sent to the Iranian people on several occasions, including for Norouz, have been very encouraging," he said. "But I think [the delisting of the MKO] would neutralize those positive statements. And it might lead to frustration with U.S. policies and even hatred."
-- Golnaz Esfandiari, Hossein Aryan