From the current popular uprisings in north Africa to the recent student marches against tuition fees to this weekend's national day of action, we seem to be in the throes of a new age of protest – one fuelled by the irrepressible power of social networking. But before the era of Twitter, Facebook, blogging and even photocopying, getting your message out was not such a straightforward business. In the golden but low-tech age of 1960s protest, with options for independent broadcasting or publishing limited, silkscreen printing was the street campaigner's weapon of choice. It was cheap, easy, fast and capable of delivering a powerful, graphic message – albeit one that came with associated risks of arrest or injury in its distribution.

The best-known of these screen-printed posters came out of the Atelier Populaire, the renegade printshop set up by students at the Ecole des Beaux-Arts during the Paris protests of 1968, but less well-known is the Atelier's London equivalent, the Poster Workshop. Its existence is barely recorded in history books and picture archives, and hardly any of the original posters survive, but for a brief spell it was at the centre of London's protest scene, and its work provides a fascinating snapshot of an era when everyone seemed to have a cause and it felt as if the revolution were just around the corner.

The Poster Workshop was set up in a basement in Camden in the summer of 1968 and for two years, until its closure, provided posters for whoever needed them, on a "pay what you can afford" basis. One of its founder members, a sculptor named Jean-Loup Msika, was a veteran of the Atelier Populaire who had been expelled by the French government, but the others were helpful locals and students. The atmosphere in London was ripe for it. The violent anti-Vietnam war protests in Grosvenor Square had happened in March, Tariq Ali's leftist Black Dwarf newspaper had launched a few months earlier, and students were starting to occupy universities and colleges demanding change.

"You could feel the excitement," says Jo Robinson, a veteran social activist who joined the Poster Workshop as an ex-art student from Blackpool. "I just jumped on board this political movement that was taking off. People were so excited, full of belief and spirit, and it felt like there was going to be a revolution any minute."

Sarah Wilson, another art student from a conservative background, who had been "radicalised" by visits to Paris, Algeria and Cuba before discovering the Poster Workshop, adds: "It was about wanting to do something more political with your training. I still knew nothing about anything really in the world of politics. But so many movements came through, you just had to be there and you learned about all these things going on around the world."

In reality, the revolution was far from glamorous. Producing 200-odd posters would take a whole day – or night, if it was urgent. As well as designing and cutting a stencil, there was a great deal of inking, squeegeeing, mopping, then hanging the posters to dry on washing lines.

"It was messy, smelly and toxic," says Wilson. "And unsafe, too. All the printers had to wear gas masks because of the solvents. By the end of the day, you'd be knee-deep in dirty rags. They were highly flammable but everybody still smoked. Robinson adds: "And in winter it got so cold, there would be icicles on the drying lines."

As word spread, all manner of groups would turn up with requests. One day it might be a local tenants' organisation complaining about unfair rent rises, the next, a group of draft-dodging American expatriates campaigning against the Vietnam war, or the "Indian Marxist-Leninist Association", or for the workers taking industrial action at the Ford plant in Dagenham. The Brixton arm of the Black Panthers would drop round and ask for a poster demanding the release of Huey Newton, their imprisoned American leader. "I dealt with a lot of the black power groups," says Wilson. "They did take advantage sometimes of our white liberal guilt, but it was very difficult for them. They were an easy target for police. But also they didn't have cars. I remember going out flyposting with them using my mother's car – little did she know!"

Some images fitted several issues, such as the iconic raised fist – borrowed from the Atelier Populaire (and later co-opted for a Chemical Brothers album sleeve). Others were designed on the spot. The images are crude and unprofessional by today's standards, but that's really the point: everything was done by hand, which explains the primitive typefaces, spelling mistakes and amateurish drawings. The posters make a clear, immediate impact with great economy.

The rhetoric often looks dangerously inflammatory now. Few young protesters would dream of walking down Whitehall today with a placard proclaiming "Students of the world, ignite!" or "Summer's here and the time is right for fighting in the streets". But it's easy to forget how much there was to rebel against, says Wilson. "It was a much more paternalistic society – very rigid, hierarchical, repressive. Young people really didn't have a lot of say in how their lives were run, especially at universities. People objected to these rigid curriculums that just reflected Britain's interests. The more they learnt about the reality of what our involvement in other countries was, the more they became interested in changing it."

Being a portable operation, the Poster Workshop even set up in student unions during occupations at the London School of Economics and others, producing posters out of the canteen. They also travelled to Northern Ireland at the start of the Troubles to produce posters on behalf of the People's Democracy movement, which supported civil rights for Northern Ireland's Catholic minority. In September 1969, Wilson set up a poster-making operation in a shop in "Free Belfast", the area barricaded by the nationalists to keep the police out. In fact, the first arrests made by the British Army during the Troubles were of three men convicted of putting up Wilson's posters and refusing to remove them (images included an armed, steel helmeted policeman in a gas mask with the slogan "Take down the barricades and let us in"). The next day the men were rapidly convicted of behaviour likely to cause of breach of the peace, which sparked heated debate in the local press. But there were still queues outside for more posters the next day, Wilson remembers.

Wilson and Robinson acknowledge how much easier it was to be politically engaged in the era of generous student grants and high employment. Nobody was much concerned about whether they'd be able to get a job or a mortgage when they needed to. Some students even boycotted exams and forfeited their degrees in protest. Both women remained active in the social movements that followed. They were both involved in the famous protests at the 1970 Miss World pageant at the Royal Albert Hall, for example, where they disrupted the show with rattles, whistles and flour bombs, and continued to be involved with communes, squatters' and council tenants' rights and other social justice issues. "We won so many things because we just stood up against authority all the time," says Robinson. "It was like a do it yourself revolution. If something wasn't right you just went and caused as much trouble as you could until you got heard. Till they wanted to get rid of you. It was brilliant."

One side-effect, however, of the Poster Workshop's screen print medium and its anarchic philosophy, is that few of the original posters survive. Having served their immediate purpose, they were routinely ripped down or posted over. To collect or preserve the posters was against their ethos. As the Atelier Populaire in Paris put it: "To use them for decorative purposes, to display them in bourgeois places of culture or to consider them as objects of aesthetic interest is to impair both their function and their effect."

Wilson and Robinson are now in search of any originals. One complete set of the workshop's posters was destroyed in a flood; another is said to be in the possession of one of the founders, Peter Dukes, though they lost contact with him 10 years ago and are still trying to locate him. Ephemeral though they were considered at the time, such posters are a document of an era of dramatic change, not just in the UK and France but around the world. They expose history's blind spots and forgotten causes, and sowed the seeds for today's street artists, such as Shepard Fairey or Banksy. But most of all, they remind us that social and political change usually begins at street level.