The National Resistance Council (NRC) is generally seen as the most important umbrella group for the Iranian opposition. An all-party statement signed this week by 250 British MPs said: "Support for the National Resistance Council, and its president-elect [Mrs Rajavi] will expedite the establishment of democracy in Iran, and contribute to the restoration of stability in the region.''
But Bonn told Mrs Rajavi, who comes from France, that she was not allowed to attend the rally because of fears she would call for the armed overthrow of the Tehran regime. German officials insisted this would be "unacceptable". Her husband, Massoud Rajavi, chairman of the NRC, said: "This soft approach towards a regime which has been condemned for its breaches of human rights and export of terrorism ... will only encourage the mullahs to further crimes.''
Mohammed Mohaddessin, of the NRC, said in Bonn that the "ludicrous" decision was the result of pressure from Tehran. He said Amir-Hossein Taghavi, a senior official in the security services, had seen foreign ministry and chancellery officials in Bonn in recent days. The foreign ministry denied it.
Germany's attitude towards Iran has long been ambivalent - seeking to stand up for human rights while being cautious not to offend Tehran for diplomatic and commercial reasons.
There was tension between Bonn and London in 1993, when Bernd Schmidbauer, co-ordinator of German intelligence, met Ali Fallahian, minister responsible for the security and intelligence services, and thus Mr Taghavi's boss. Mr Fallahian was described by Britain at that time as "the man responsible for terrorism".
In other respects, too, German attitudes have been confused. Following criticism, Klaus Kinkel, the German Foreign Minister, finally agreed to a meeting in April with Salman Rushdie. Meanwhile, Lufthansa refused to carry Mr Rushdie for security reasons, which led to protests from German MPs. Mr Kinkel intervened, with a letter to the airline asking it to change its policy. Lufthansa refuses to comment.
Tomorrow's protest, in Dortmund, looks set to be one of the biggest in recent years, with 20,000 expected. Germany has Europe's biggest Iranian community, at more than 100,000.
Critics of the NRC, which is linked to the rebel mujahedin, say it is in effect still a terrorist organisation. Others say things have changed recently. An observer said yesterday: "The mujahedin want to please the West. They wouldn't do anything right now that might get them labelled as terrorists.''