Thousands of supporters of Catalan independence marched on Madrid to support politicians and activists who are on trial for their role in an attempt to secede from Spain.

Organisers said 120,000 people marched in Spain’s capital, while Spanish police put the number much lower at only 18,000.

Twelve Catalan politicians and activists are standing trail for staging a banned independence referendum for the northeastern region of Catalonia.

Nine of those on trial are accused of rebellion, which could see them sentenced to up to 25 years in prison.

The October 2017 referendum passed with overwhelming support, with 92 per cent in favour and a turnout of 43 per cent of the electorate.

However, Spain had already declared the referendum unconstitutional and some observers said the vote failed to meet international standards for elections.

Large pro-independence protests have previously taken place in Catalonia and some European cities, but Saturday’s march was the first major separatist protest in the capital.

The 120,000 crowd size quoted by organisers was well beyond expectations.

There were no reports of violence at the march.

A banner reading “Self-determination is not a crime” opened the demonstration, which was led by Catalan politicians and organised by more than 60 civil society groups across Spain.

Catalonia’s regional president, Quim Torra, told reporters that the march was needed to decry a trial which he has described as a “farce”.

Spain has insisted the judicial process is fair and its courts are fully independent from the government, but critics of the trail believe the 12 defendants are political prisoners who are being tried for their beliefs.

The trial is about to enter the fifth week of hearings and has attracted close attention across the country.

Media have been broadcasting lengthy live testimonies by defendants and witnesses from the trial and radio stations have hosted in-depth debates about prosecution strategies.

A sentence is not expected until after Spain’s 28 April general election, in which the issues of Catalan independence and Spanish unity have become major topics for voters.