Two Danish activists who took part in the Copenhagen climate demonstrations last December have been found guilty of organising and instigating violence and vandalism, and have both been given four-month suspended sentences. One of the three judges in the case disagreed with the verdict.

Tannie Nyboe and Stine Gry Jonassen were both spokespeople for the Climate Justice Action group, part of the network involved in some of the demonstrations in Copenhagen during the UN's COP15 climate summit. They have been convicted of four charges, including inciting violence against the police, serious disturbance of the police, interfering with police in the course of their work and destruction of property.

The case against them was based, controversially, on evidence gathered by tapping their phones before the conference, and also on video footage taken during the "Reclaim Power" demonstration on 16 December.

On that day, several thousand demonstrators gathered outside the Bella Centre where the conference was taking place, with the stated aim of getting as near as possible to the centre and meeting with delegates, in order to hold an alternative "people's summit". During a scuffle with police, Nyboe and Jonassen were filmed standing on a lorry shouting "push" into microphones. Over the course of the trial their supporters posted photographs of themselves holding signs reading: "I too shouted push!"

The deputy chief prosecutor, Jakob Buch-Jepsen, told the Guardian: "We are very pleased the court has agreed with our case. Before this demonstration we were told it would be non-violent, but at the same time they said they wanted to go into the area outside the centre. They wanted to go through the police cars and the policemen, and from my point of view if you have 2,000 people pushing against the police, the police are in very big danger."

Defence lawyer Thorvild Hoyer said: "I think it's a very unfortunate verdict. How is it possible to convict two people of organising violence when no one was actually convicted for violence? Nothing really happened that day. Nyboe and Jonassen were willing to act as spokespersons, and try to be as open as possible with the police. But I think people [activists] will now be more anonymous." He confirmed that one of the three judges on the case had voted to find the defendants not guilty.

Tannie Nyboe said: "I am really shocked about the outcome and didn't expect it at all, and now we're trying to figure out what to do next. Looking back I'm still so surprised about the way the police behaved during the whole COP, it seemed like they wanted to stop people protesting rather than allowing people to protest peacefully."

After nearly two thousand arrests during COP15 last year, there have been only two other minor convictions, related to a different demonstration. Prosecutors have still not decided whether to formally charge the Greenpeace protesters who got into a state banquet. The police prosecutor confirmed that charges of "insulting her majesty" were unusual in Denmark, but that they are still being considered.

A decision on a separate case concerning 159 formal complaints filed against the police will be announced on 16 December.