By Jim Muir BBC Tehran correspondent

A leading figure in the Iranian opposition movement, Ebrahim Yazdi, has returned to Tehran after spending 18 months receiving cancer treatment in the United States.

Many of the followers of his Freedom Movement were arrested last year on charges of plotting to overthrow the Islamic regime.

Mr Yazdi himself faces a summons from the revolutionary court.

I'll have no political retirement. I will continue the previous policies. My poor health cannot prevent me from fulfilling my responsibilities to my people

Ebrahim Yazdi

There were tearful scenes at Tehran Airport as Mr Yazdi, 70, stepped back onto Iranian soil.

Many of his supporters who turned out to meet him must have thought they might never see him again.

He has lost weight, he has lost his hair - but evidently he has not lost his defiant spirit.

Mr Yazdi knows he faces a summons from the revolutionary court to answer charges of threatening national security and possessing illegal arms - charges he strongly denies.

Last year, about 40 of his associates were arrested on similar charges, including plotting to overthrow the regime.

The case against them continues. But they have all been released on bail.

Opposition hero

The question now is whether Mr Yazdi himself, who has vowed to keep up his political activity, will be arrested.

He told the BBC he did not know what would happen, and that anything was possible. He said he was still under medical treatment and needed constant supervision.

Clearly, the judiciary risks making a hero out of Mr Yazdi if it does proceed against him.

The prosecution of members of his Freedom Movement and other figures in the so-called national religious opposition brought widespread denunciation from human rights groups - both at home and abroad, as well as from reformists within the Iran system.

They do not see peaceful liberal opposition as any kind of threat.

Mr Yazdi was a close associate of the leader of the Iranian revolution, Ayatollah Khomenei, and he served as foreign minister in the first revolutionary government.