Thousands are in central Madrid for a protest by centre-right parties opposed to a plan by the Spanish government to ease tension in the Catalonia region.
The Popular Party and Cuidadanos (Citizens) called the protest after PM Pedro Sanchez said he would appoint a rapporteur for talks with separatists.
They consider the appointment a betrayal and surrender to separatist pressure, and want early elections.
Like the right, the ruling Socialists also oppose Catalan independence.
Far-right groups including the Vox party are also present at the protest, held under the slogan “For a united Spain. Elections now!”
Catalan nationalists regained power in Barcelona in May, after a seven-month period of direct rule by Madrid.
Tensions remain high, as many Catalans resent Madrid’s show of force last year, when it charged pro-independence leaders with sedition.
Mr Sánchez heads a minority government that depends on nationalists – including Catalans – to stay in power, but he has ruled out any new Catalan referendum on independence.