Ugandan opposition leader Kizza Besigye has held a series of political rallies a day after being released on bail from detention over terrorism charges.
Dr Besigye was once President Museveni's personal physician
About 10,000 of his supporters turned out to hear Dr Besigye campaigning north-east of the capital, Kampala.
Dr Besigye criticised President Yoweri Museveni over democracy and rights and said it was time for change in Uganda.
President Museveni has been campaigning in western Uganda ahead of February's presidential election.
He described Dr Besigye as a coward for leaving the army.
Dr Besigye is due to appear in court later this week to face further charges of treason and rape. He has denied the charges saying they are politically motivated.
He was arrested in November after returning home from four years' exile to contest the polls.
The BBC's Will Ross said his first rally was almost a disaster for Dr Besigye as the stage nearly collapsed and there were problems with the sound system meaning many in the crowd could not hear him.
Speaking earlier at a news conference, Dr Besigye said he had been held illegally and that President Museveni should be the one in court.
This was proved by a world court ruling that Uganda was wrong to invade the Democratic Republic of Congo, he said.
The International Court of Justice ruled last month that Uganda must pay compensation to DR Congo for looting during the 1998-2003 war and the two countries are negotiating on the level of compensation.
"Taking our troops to Congo without the authority of parliament as required by the constitution where thousands of our soldiers died. What more treason can there be?" Dr Besigye asked a news conference at his Kampala home before heading east of the capital to attend a campaign rally.
"It should be Museveni in the docks and not Kizza Besigye."
President Museveni has been in power since 1986
Dr Besigye is viewed as the first credible challenger to Mr Museveni, who has been in power for 19 years.
Three other presidential candidates are contesting next month's polls.
Democratic Party candidate Ssebana Kizito turned up in the middle of Dr Besigye's news conference on Tuesday, prompting speculation that the two may join forces.
These will be the first multi-party elections since Mr Museveni took power.
Ugandan presidential spokesman John Nagenda told the BBC he viewed Dr Besigye's release as a very good thing, as it meant he would not now get any sympathy votes in the presidential election.
"It actually says a lot for the government that they haven't tried to in any way control the judgements of the High Court," he said.