Basque nationalists have been banned from launching a new political party after a Spanish judge said it was a cover for the banned group Batasuna.
Batasuna leaders are seeking a way back into politics
The new party was due to be launched at a rally on Saturday. Supporters said the meeting would go ahead despite the ruling by Judge Baltasar Garzon.
Batasuna is banned in Spain for its links with the armed group Eta.
But its members have been seeking a way back into politics before May's local elections, a BBC correspondent says.
They are planning to launch their election programme in Bilbao on Saturday, under the new name, Abertzale Sozialisten Batasuna, or Patriotic Socialist Union.
But Mr Garzon, Spain's top anti-terror judge, has ruled that the meeting is illegal, on the basis that the new group is apparently a continuation of Batasuna.
'Attack on freedom'
Spain's supreme court is expected to rule soon on whether Abertzale Sozialisten Batasuna should be banned outright.
Prime Minister Jose Luis Rodriguez Zapatero has already said there are indications that it is illegal.
The government has said Batasuna can only participate in elections if it disavows violence once and for all.
Eta broke a nine-month ceasefire with a bombing in December that killed two people.
But Batasuna leader Arnaldo Otegi said the ban was "a serious attack on democratic freedoms", and a spokeswoman said the meeting would go ahead.