Former Deputy Finance Minister Nadia Valavani joined Popular Unity on Friday.

One small faction within the group has even suggested it might join the breakaway far-left Popular Unity, Syriza officials say.

Lapavitsas, the professor, acknowledges that Popular Unity has a long road ahead and its work cut out to win public trust.

The march, called by the Basques' Popular Unity party, demanded that jailed pro-independence guerrillas' be transferred to prisons in their home region.

The only other explicitly pro-independence grouping, the radical left Popular Unity Candidacy (CUP), took 8 per cent of the vote.