The raids were in connection with a demonstration held on June 20 against the multi-billion-euro project. Protesters stormed a building site and occupied the site for five hours, allegedly damaging fences and construction equipment.
The office of the group Parkschützer, as well as the home of spokesman Matthias von Herrmann, were searched for video footage and information about potential witnesses to an alleged attack on a policeman that took place during the demonstration, officials said.
According to police and Germany's national rail operator Deutsche Bahn, the demonstration caused €1.5 million in damage. An undercover police officer allegedly suffered injuries to the head and neck caused by punching and kicking.
The state prosecutor is investigating the incident and may lay charges of causing bodily harm and attempted robbery.
Police were searching for video footage that may show the alleged assault, as well as evidence of who might have witnessed the incident, officials said.
The Parkschützer group showed video material at a recent press conference but have not handed it over to police, officials said. Von Hermann and other possible witnesses also ignored a police summons, forcing police to carry out the raids, they said.
Von Herrmann said it was an “absolute affront” for the police to claim that Parkschützer had refused to hand over material. The video that the group showed two weeks ago at its press conference was available for everyone to see on the website Youtube, he said.
“This is a specific manoeuvre to portray the Parkschützer as criminals,” he said.
Stuttgart 21 consists of a massive construction effort, involving rebuilding the city's main train station underground and turning it around 90 degrees, as well as laying 57 kilometres of new tracks. The aim is to make the city a major European rail hub.
The official price tag is capped at €4.5 billion but some media reports suggest the true cost may be higher. Opponents have mounted massive protests against the project over the past year, calling it too expensive and unnecessary.