The Ugandan government on Wednesday outlawed the activist group that has since last year led popular protests against official corruption and the high cost of living, saying it had become a force for instability.

The ban against Activists for Change was issued by Uganda's attorney general, a government spokesman said. It sets up a potential confrontation for Thursday, when the group planned to hold another march.

"It has been declared an illegal group," said Fred Opolot. "Over the past few months they have organized rallies and processions without due consultation with the police. Most of the processions have turned out to be violent."

The decision to criminalize the group was announced on the eve of a planned demonstration in the Ugandan capital of Kampala, which is hosting parliamentarians from all over the world.

The group's leaders include Uganda's top opposition politician Kizza Besigye, who has been repeatedly arrested for his role in the so-called "Walk to Work" protests. The walks have recently turned violent, with the police using force and tear gas to break up processions they say disrupt business in the city.

Anne Mugisha, an activist and a Besigye associate, said the decision was "unconstitutional" and that the group would continue its activities. The group said it would hold its first post-ban rally on Thursday.

"We are going to challenge this decision in the Constitutional Court," Mugisha said. "It is going to fall flat on its face. Our campaign is a campaign of defiance. We are not seeking anyone's permission."

Recently a policeman was killed in violent street clashes between Besigye's supporters and the police. Officials say the policeman was felled and killed by a rock thrown by one of the demonstrators last month.

Ugandan President Yoweri Museveni said in the aftermath of the killing that those responsible would "pay dearly." Two days later police and security officials responded by arresting 100 suspects in a swoop that some critics said was reminiscent of the fearsome tactics employed by former Ugandan dictators. Besigye and some of his associates were charged last week with covening an unlawful assembly in a case stemming from the death of a policeman.

Uganda's opposition accuses Museveni of becoming increasingly dictatorial, and some parliamentarians have initiated an effort to impeach him over graft and what they say is his mismanagement of the economy.

"There are enough reasons to impeach President Museveni 10 times over," said Fred Bbosa, vice president of the Uganda People's Congress, one of the country's oldest parties.