Democratic Alliance MPL Jack Bloom said office security detained Johannesburg High Court deputy sheriff Diana Chivelli on the third floor of the building.
She was at the office to collect a portion of the R9.25m owed by Mokonyane for the medical costs of a boy left brain-damaged because of negligence in a state hospital.
"The sheriff is there to remove the furniture... as the premier has failed to respond to the court payment order. Her lawyers appealed the judgment, but this appeal has lapsed," Bloom said.
"It is an utter disgrace that the premier has failed to obey this court order that would assist Khanyi's struggling family to look after him."
Mokanyane's spokesperson Xoli Mngambi denied that Chivelli had been detained.
"We do not agree with what Bloom is saying. She is not being detained. They [Mokonyane's office] have been in talks about the situation since the sheriff got there," he said.
"Our legal department, along with the department of health and the state's attorney, are dealing with the matter."
The furniture at Mokonyane's offices was attached by the sheriff in October last year.
Bloom said 270 computers, 270 desks, 600 chairs and other office equipment, and kitchen appliances worth about R1m were attached.
Twelve-year-old Prince Sibusiso Khanyi was born brain-damaged at the Pholosong Hospital in the East Rand in December 1999.
The South Gauteng High Court in Johannesburg found in February last year that he suffered permanent brain injury because of hospital negligence.
It ruled that Mokonyane had to pay his parents, Martha and Simon, nearly R10m in damages.
Bloom said Mokonyane should "obey the law just like everyone else and pay the full amount in good time".
"It's a very sad day for our province when the premier defies the law," he said.
"Meanwhile, a disabled child suffers because her office has delayed in paying money that would ease his life and that of his family."