Ugandan opposition leader Kizza Besigye has been prevented from leaving his house by police, as the new parliamentary session is set to open.
The police say that they had information he had mobilised supporters and was about to commit a crime.
He has been arrested four times in recent weeks during opposition protests at the rising cost of living.
Human rights groups have criticised the police crackdown on the protests, which left nine people dead.
The BBC’s Joshua Mmali in Kampala says Dr Besigye left his house in a car and was stopped by a police roadblock.
They said he was under “preventive arrest” and gave him two options – either to return home or to go the police station.
He went home and his party says he is effectively under house arrest.
Dr Besigye says he was cheated in February’s election, although he mustered only half as many votes as President Yoweri Museveni.
The government accuses him of using trying to organise an Egypt-style uprising.
Changing the law
From his house on Friday morning, he told the BBC’s Network Africa programme he felt ashamed that Uganda had “regressed back to the dark days of previous atrocious dictatorships”.
“When we used to have a law called the preventative detention act under which political opponents could be detained indefinitely by the regime in power,” he said.
“I have no idea what the conditions of my house arrest are… they didn’t say for how long.”
He says he has instructed his lawyers to challenge the police action in court.
As the new parliament session begins, President Museveni wants MPs to urgently change the law so that people accused of crimes, including rioting and economic sabotage, are not entitled to bail for six months.
BBC East Africa correspondent Will Ross says this is widely seen as an attempt to lock up Dr Besigye.
Dr Besigye, a former ally of President Museveni, sought medical treatment in Nairobi after being injured when he was arrested during the protests.
Last month, riots broke out in Kampala in protest at the rough treatment meted out to Dr Besigye by the security services during his arrest on 29 April.
Plain-clothed policemen beat up his supporters, smashed the window of his car and doused the inside with pepper spray and tear gas before manhandling him into a vehicle and driving off.
The authorities say Dr Besigye provoked them – and he was charged with inciting violence.
Mr Museveni came to power in 1986, following years of misrule under Milton Obote and Idi Amin.
He was initially praised as part of a new generation of African leaders determined to raise living standards and bring democracy to the continent.
But recently, donors have cut off some aid amid concern about his commitment to human rights.