It was a picnic that changed the course of history.

Twenty years ago on Wednesday, members of Hungary’s budding opposition organised a picnic at the border between Hungary and Austria to press for greater political freedom and promote friendship with their Western neighbours.

Some 600 East Germans got word of the event and turned up among the estimated 10,000 participants. They had a plan: to take advantage of an organised excursion across the border to escape to Austria.

Hungarian President Laszelo Solyom and German Chancellor Angela Merkel took part in the festivities on Wednesday marking the 20th anniversary of the “Pan-European Picnic”, which helped precipitate the fall of the Berlin Wall nearly three months later. One of the key factors allowing the Germans to escape: the decision by a Hungarian border guard commander not to stop them as they pushed through to freedom.

Lt Col Arpad Bella and five of his men had been expecting a Hungarian delegation to cross the border at Sopronpuszta by bus, visit a nearby Austrian town as a symbol of the new era of glasnost — or openness — under reformist Soviet leader Mikail Gorbachev, and return to Hungary. Instead, at the planned time of 3 pm, Bella suddenly found himself face to face with 150 East Germans marching up the road to the border gate, which had been closed since 1948. “I had about 20 seconds to think about it until they got here,” said Bella, 63, during an interview where the gate once stood. “Had the five of us confronted the Germans, they would have... So why did they send us here with loaded guns? A slingshot or holy water would have been just as well.”

Once the group got through hundreds more East Germans joined them. Still vivid in Bella’s mind was the reactions of the Germans, including many young people and families with small children, once they were on the other side. “They embraced, they kissed, they cried and laughed in their joy. Some sat down right across the border, others had to be stopped by the Austrian guards because they kept running and didn’t believe they were in Austria,” Bella said. Laszlo Nagy, one of the organisers of the picnic, was startled by the East Germans’ actions, who left behind hundreds of cars and other possessions near the border.

While Bella was unaware of the East Germans’ intentions, behind the scenes the Hungarian government had already decided that it would somehow let them go West. Miklos Nemeth, Hungary’s last prime minister of the communist era, said the picnic and the East Germans’ breakthrough on that day was one in a series of steps that brought democracy to most of the Soviet bloc within a year.

“It was a planned process on behalf of the government, but it was a transition where everyone was also seeking to secure their own future,” Nemeth said.