Carlos Montes was asleep in his Los Angeles home when his door exploded into splinters at five o'clock in the morning.

Terrified he was being robbed, the 64-year-old called out "Who is it?" as armed men wearing bulletproof vests rushed into his bedroom, assault rifles aloft and torches shining in his eyes.

It was the police. Montes, a longtime leftist activist turned retired Xerox salesman, was handcuffed and taken outside. It was then he saw the scale of the operation deployed to arrest him. Two Swat vans stood parked nearby and about 20 armed police milled around in the street.

He was put in the back of a squad car and a sheriff explained they had discovered that Montes – whom had long been registered as gun-owner – had a previous felony conviction. It dated from 1969 and was for throwing a soda can at a policeman during a civil rights demonstration. That was enough to render Montes' current possession of guns illegal as state law held convicted felons could not own them, the sheriff said.

"I yelled 'What?'," Montes told the Guardian in an interview, expressing incredulity as to why two Swat teams needed to smash through his front door and confiscate cellphones, a laptop and all his computer memory sticks. "I am 64 years old and they know about the guns because I always registered them," he said.

Then another man, in plain clothes and wearing a baseball cap, got in the car and explained what Montes and a growing legion of supporters believe was the real reason for the massive night-time police raid. "He said: 'I am from the FBI and I want to talk to you about the Freedom Road Socialist Organisation.' Then I knew this was political. It is all political," Montes said.

Montes is now due to go on trial on 20 June for illegally possessing guns. If found guilty he could face 22 years in jail. His case has prompted a national campaign to support him with rallies, petitions and lobbying of politicians. His name has become synonymous with accusations the FBI is using its investigative powers to tackle increasingly political causes. It has particular significance after undercover FBI informants recently used so-called entrapment techniques against activists in Cleveland and Chicago.

Prosecutors in those cases say they broke up dangerous anarchist cells bent on violence. Defence lawyers say they were deliberately goaded into criminal acts by FBI plants.

Montes's supporters insist he is being victimised for a lifetime of political activism, including links to the otherwise obscure FRSO and other fringe left-wing protest groups. The FRSO, of which Montes says he is not even a member, was among many groups who protested the Republican convention in St Paul in 2008. Montes also demonstrated there, speaking at rallies and taking to the streets in protests.

At the same time a group called the Anti-War Committee, which was linked to the FRSO, was infiltrated by an FBI undercover agent posing as a lesbian single mother and calling herself Karen Sullivan. A series of FBI raids in 2010 netted numerous members of the Anti-War Committee and a list of targets desired by the FBI emerged. Montes's name was on it.

In response to the arrest of his fellow activists, Montes then set up an anti-FBI website called the Committee to Stop FBI Repression. In light of what happened to him, that might not have been wise. But then Montes has rarely shied away from a fight with authority.

Montes, who is American-born but the son of Mexican immigrants, was a leading light in a 1960s group known as the Brown Berets, a sort of Hispanic equivalent of the Black Panthers. He was a fixture of 1960s protest culture whose activism eventually put him at the heart of seminal cases such the "East LA 13" – who had organised a California college walkout – and the "Biltmore Six" – for an alleged arson at a hotel where then California governor Ronald Reagan was staying.

However, despite such a high profile, one count of throwing a soda can at a police officer (who was not injured) was Montes's only conviction. Indeed, as the 1970s wore on the times changed and Montes eventually got work as a Xerox salesman. For 20 years, during which he became Xerox's top salesman for downtown LA, Montes was an activist during the weekends and the evening.

Yet now he faces more than two decades in jail stemming from a sole conviction 43 years ago. There certainly seem to be many strange things about the sudden police interest in Montes. Firstly, police were well aware of his guns as Montes had registered them, unaware his 1969 conviction was a problem. "I felt I had the legal right to own guns," he said. So, apparently, did the police.

Following a 2005 argument with a then girlfriend, police took away his guns while they investigated a domestic dispute. They then gave them back to Montes once charges were dropped. The necessity of raiding Montes' home during the middle of the night also stands out as being unusual. Yet it was specifically requested by police. In a handwritten note at the bottom of the search warrant Sheriff Don Lord wrote Montes' "anti-government ideology" meant he posed a risk and thus Lord requested a "night service" raid.

That same warrant, a copy of which has been seen by the Guardian, also gave police the legal power to confiscate all computers and electronic devices in Montes's home. The justification was that Montes may have scanned and uploaded his gun receipts electronically. Of course, those very same computers also detail all of Montes's political activities and contacts.

Meanwhile, an incident report of the raid, also seen by the Guardian, specifically reveals it was the FBI who discovered Montes's guns might be illegal, effectively prompting local police to act on a four-decades-old incident.

Certainly Montes has no doubt the FBI is seeking to punish and silence him for his political activities. He is also concerned that he faces spending the rest of his life behind bars. "Of course, I worry about it a little. It's a long time," he said. One legal strategy is to show that his 1969 conviction was not actually a felony, but a misdemeanour. But the records are so old the exact status of the conviction is not clear. Still, Montes says he will not be silenced.

"They want to scare everybody, but we are not going to take it lying down. We are going to fight back," he said.

It is perhaps worth pointing out Montes maintains he never even threw the soda can back in 1969 that is now the cause of his troubles. He says he was arrested hours after the demonstration was over and the sole evidence against him was the word of one police officer. "That was a set-up back then too," Montes said.