The stark warning comes as tens of thousands of protesters gathered across an increasingly polarised Spain calling for national unity and negotiations.
Spain’s constitutional court has ordered the suspension of the Catalan parliament tomorrow as Madrid tries to buy time.
However, Catalan leader Carles Puigdemont is expected to call for independence if the chamber is allowed to resume on Tuesday.
It follows a banned referendum last week in which 90 per cent – 2.2 million voters – backed independence on a 43 per cent turnout.
A further 750,000 votes were confiscated by police and have not been counted.
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Former British commercial attache Mike Thom said: “I’ve lived in Barcelona since 1999 and I can’t believe what I’ve been seeing.
“Even before the referendum we witnessed paramilitary police wearing full riot gear raiding public buildings just to seize files as they tried to stop it taking place. And on Sunday the whole world saw police beating women and children.
“Three weeks ago less than half the population of Barcelona would have voted for independence. But the brutality used by police, with the express permission of Madrid, has changed this.”
He added: “No one knows what will happen if the Catalan parliament declares independence on Tuesday but I can tell you we are all very frightened.
“There will be troops on the streets. Tanks that have already been transported here and are being kept away from the public will be rolled in.
“Barcelona will become another Tiananmen square and while Catalans have been extremely peaceful, young people – students who have seen their grandmothers beaten up in the street – are getting more and more angry.”
There was little sign of compromise in Madrid last night, a mood not helped by incendiary government declarations that Catalan politicians would face the full wrath of the law if they declared independence.
In Barcelona, protesters chanted “Let’s talk” in Catalan, while many carried signs criticising political leaders for not finding a diplomatic solution to the impasse.
In Madrid thousands gathered beneath the enormous Spanish flag in Colon Plaza waving their own flags and chanting “Viva Espana” and “Viva Catalonia”.
Catalan leaders said they remained open to negotiations. Prime Minister Mariano Rajoy has indicated this can only happen if Mr Puigdemont gives up the independence campaign.
Sergi Marcén, leader of the Catalan delegation to Britain, said yesterday: “Talk of civil war is ridiculous – Spain knows what civil war means – and we Catalans have always resisted peacefully.
“But we are angry.
“And the way in which we were treated by Madrid, with brutality while we kept our arms by our sides, has made the rest of Spain angry with our incompetent government.
“The events of Sunday, where more than 100 people were injured, happened after years of trying political dialogue. We were beaten. One man had a heart attack. As his daughter went to help, police hit her. Where is the humanity?
“If the Spanish government doesn’t talk to us, it will push us to a place where we don’t want to go.
“We’ve tried 18 times to come to the negotiating table in the past five years. It has proved to be impossible.” He condemned King Felipe VI of Spain for failing to criticise the police when he addressed the nation on Tuesday, tearing up any chance of Spanish-led mediation.
Mr Marcén said: “King Felipe had a golden opportunity to bring both sides together. Instead he alienated all of Spain with a speech that could have been written by Rajoy himself.
“Only international mediation can have any hope of working now.”
While Germany’s Chancellor Angela Merkel has held talks with European Commission president Jean-Claude Juncker over the crisis, Brussels still views the issue as an “internal affair”.
Mr Marcén said: “They are just sitting on their hands. It stopped becoming an internal affair when Spanish police committed human rights violations. We Catalans love Europe.
“We thought the EU represented the people of Europe, not just its states. It seems we were wrong.”