The Ugandan government has banned the opposition Action For Change group, which has staged many protests against corruption and the cost of living.
The group has become known for its "walk to work" demonstrations, during which its leader Kizza Besigye has faced frequent arrests.
The government accused it of organising protests without consulting police.
The ban comes on the eve of a march planned for the capital Kampala.
The decision to declare Action For Change an illegal group was authorised by the Attorney General following a cabinet meeting.
"Over the past few months they have organised rallies and processions without due consultation with the police," government spokesman Fred Opolot told Associated Press.
"Most of the processions have turned out to be violent," he added.
Action For Change was planning to hold a march on Thursday in Kampala, to coincide with a meeting this week of the Inter-Parliamentary Union.
The "walk to work" protests started following elections in February 2011 which the opposition said were rigged.
Mr Besigye lost the presidential vote to President Yoweri Museveni for a third time. He was the president's doctor before the pair fell out.