Video report by ITV News Correspondent Dan Rivers
More than a half million protesters, including families with children, marched in the Catalan capital, according to local police.
Many were clad in pro-independence 'estelada' flags and shouted "Independence!" and "Freedom for political prisoners!"
Some of them had walked for three days in five massive "freedom marches" from towns across the northeastern Spanish region.
They converged on Barcelona, a city of 1.6 million people, and joined students and workers who also took to the streets during a 24-hour general strike.
Around 400 people, roughly half of them police officers, have been injured according to regional and central authorities, and 128 arrested since separatist sentiment surged on Monday, when the Supreme Court sentenced to lengthy prison terms nine separatist politicians and activists.
The nine had led a 2017 push for independence that triggered Spain's deepest political crisis in decades.
This week's huge show of support at times turned violent, with some protesters and riot police fighting running battles.
On Friday, the demonstrations quickly descended into violence.
Police clashed with a few hundred young protesters who hurled bottles, eggs and paint at the gates of the police headquarters in the center of the city.
Large trash containers were burned before police responded, using rubber bullets and tear gas to disperse the crowds.
The separatist movement is proud of its history of mostly peaceful campaigning. Officials have accused a relatively small number of agitators of provoking the recent riots.
Spanish authorities suspect a secretive new group called Tsunami Democratic is using encrypted messages to orchestrate some of the attacks, which have included torched cars and burning barricades in the streets.
The group appeared on September 2 and in just over six weeks has gained nearly 340,000 followers on its main channel in Telegram, a messaging app.
A National Court judge on Friday ordered the closure of websites linked to the group.
Spain's interim interior minister, Fernando Grande-Marlaska, defended the police action as "proportionate" and warned Catalan separatists that Spain will apply the criminal code "with all its force," threatening them with prison terms of up to six years.
Tourists also felt the turmoil. At least two large cruise operators diverted their ships to other ports, and those which had already docked in the port of Barcelona cancelled their passengers' excursions to the city.
Architect Antoni Gaud''s modernist Sagrada Familia also closed its doors due to a protest blocking access to the basilica.
Naoya Suzuki, a 34-year-old tourist from Japan, complained about the disruptions to "people who have nothing to do with Spain."