Uganda accused the US ambassador in the country of seeking to subvert last week's presidential election.
The accusation came after the American embassy in Kampala said ambassador Nathalie Brown was prevented from visiting and speaking to opposition leader Bobi Wine at his home, which has been surrounded by security forces since the vote.
President Yoweri Museveni, in power since 1986, was declared winner of the poll although Wine and his National Unity Platform want to legally challenge the results.
Mr Museveni claims the election may turn out to be the most "cheating free" in Uganda's history.
The US embassy said the vote was tainted by harassment of opposition candidates, suppression of media and rights advocates and a nationwide internet shutdown.
It said Ms Brown visited Wine's compound to check on his health and safety.
But government spokesman Ofwono Opondo claimed, without providing evidence, that Ms Brown had a track record of causing trouble in countries where she worked.
"But what she has been trying to do blatantly is to meddle in Uganda's internal politics, particularly elections, to subvert our elections, and the will of the people."
The public rebuke of the US is relatively unusual because the two nations are allies.
Washington supports Ugandan soldiers serving in an African Union peacekeeping mission in Somalia and has donated about $1.5 billion to Uganda's health sector in the past three years.
Troops have prevented Wine – real name Robert Kyagulanyi – from leaving his home since he returned from voting on Thursday.
On Tuesday, he said he and his wife had run out of food, and milk for their niece. Her father has been refused entry to collect the 18-month-old, Wine wrote on Twitter.
Day Six under house arrest and we're still stuck with an 18 months old baby who had paid a visit to her auntie (my wife ) be4 we were raided & besieged. The Dad has been denied access to her. We have run out of food and milk. No one is allowed to leave or come into our compound. pic.twitter.com/KVLDR3IH6y
— BOBI WINE (@HEBobiwine) January 19, 2021
A police spokesman said a motorcycle courier had delivered food to Wine's house each day.
On Tuesday Wine's lawyers filed a petition in the high court challenging the legality of detaining Wine and his wife without charge.
