A married couple with Iranian roots was arrested "just in time" as they allegedly prepared to head to France from Belgium to bomb a large rally an Iranian opposition group held over the weekend, authorities said Monday.

The federal prosecutor's office said the Belgian citizens were charged with "attempt at terrorist murder and preparing a terrorist crime" against the opposition group, the Mujahedeen-e-Khalq. U.S. President Donald Trump's personal lawyer, Rudy Giuliani, was among the speakers at the Saturday rally in the town of Villepinte near Paris.

Investigators who detained the couple on Saturday and searched their car found about half a kilogram (more than a pound) of TATP explosives and a detonator, federal magistrate Eric Van der Sijpt said.

A half-kilogram of TATP could cause a sizeable explosion, and in a crowd estimated to have reached 25,000, considerable bloodshed and widespread panic.

"We can say we arrested them just in time," Van der Sijpt told The Associated Press.

As well as Iranian exiles, there were several American speakers at the event, which took place in a packed hall at an exhibition center north of Paris.

"All I can say is that they were planning a terrorist attack at a conference. I don't know if they were targeting somebody special," Van der Sijpt said.

The Belgian prosecutors' office said Belgian security services, France's domestic intelligence service and German judicial authorities had been instrumental in preventing another terror attack in France.

Police raided five homes after the couple was detained, but did not disclose if they found additional evidence.

A man detained in Germany "seems to be an Iranian diplomat attached to the Iranian Embassy in Vienna, in Austria," Van der Sijpt said.

Three people also were detained for questioning in France. Investigators worked to determine their relationships with the Belgian suspects, according to a French judicial source who discussed the ongoing investigation on condition of anonymity.

The Mujahedeen-e-Khalq, widely known as MEK, is an exiled Iranian opposition group detested by the Islamic Republic of Iran, which refers to them as "hypocrites."

The formerly armed group had been on European Union and U.S. terrorism lists before being delisted from both. The weekend rally, called "Free Iran 2018 — The Alternative," sought to plant the seeds for regime change in Iran.

"Regime change in Iran is within reach as never before," leader Maryam Rajavi said at the event.

Trump's recent appointment of John Bolton, who spoke at a MEK rally before he became national security adviser, and the president's choice of Giuliani, a regular MEK headliner, as his lead lawyer, are seen as moves that could potentially strengthen the group's bid for U.S. backing.

TATP, the explosive Belgian authorities said was found in the couple's car, has been used in several terrorist attacks in Europe in recent years, including the 2016 bomb attacks in Brussels which killed 32 people.

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Elaine Ganley and Lori Hinnant in Paris and Frank Jordans in Berlin contributed to this report.