Ugandan police and military forces armed with door cutters, AK-47 guns, tear gas canisters and batons stormed offices of the National Unity Platform party in Kampala on Wednesday.
Roads leading to party offices were cut off as police parked huge trucks in front.
Security officers forced open doors and took items from inside. They later asked members of the party to sign for what was taken.
Joel Ssenyonyi, the NUP spokesperson, said the officers took items from the office of party leader Robert Kyagulanyi, better known by his stage name Bobi Wine.
"They have taken paraphernalia, that is red berets. T-shirts. They have taken documents ... we are still trying to establish exactly," Ssenyonyi said. "They also broke into honorable Kyagulanyi's office, which had many sensitive documents. We were preparing to take his signatures that we have collected from different parts of the country to the Electoral Commission on Friday and many of those documents are now missing."
In a written statement, Enanga Fred, the Uganda Police spokesperson, said police are targeting all locations illegally manufacturing, supplying, distributing, selling and using uniforms and accoutrements whose ownership and patented designs belong exclusively to the armed forces.
Police have not responded to accusations that they took election-related documents during the raid.
Ugandan political analyst Anna Mary Nanfuka said the raid was ill-advised, and will promote the NUP party, while making President Yoweri Museveni look "paranoid."
"It just gives NUP more visibility and more publicity. And, you know the youth, your allegations are not going to change anything they think about NUP or People Power. So, they will look at you like 'aahh, then there is something there.' But it looks like the big man fears them more than he should," Nanfuka said.
Aspiring candidates for parliament are due to present their credentials and nomination fees to the Electoral Commission beginning Thursday.
It was not clear how candidates from the National Unity Platform will submit their forms if police have their documents.
Uganda goes to the polls early next year to elect new legislators and the next president. Bobi Wine is expected to challenge Museveni, who has ruled Uganda for 34 years.