Reporters from the Zimbabwean newspaper Newsday found them uprooting lemon trees, digging shafts and loading gold ore into lorries.
After unsuccessfully confronting them on Thursday, Mrs Mugabe reported the matter to the police
The farm is in Mazowe, where she had forcefully evicted villagers in 2015 – while her husband was still in power.
Mrs Mugabe said in a statement to the police obtained by the New Zimbabwe website that she was touring the farm when she was “shocked to find approximately 400 men illegally panning for gold”.
When she confronted them at the farm, some 40km (25 miles) north of Harare, she said they “started to shout obscenities at me and continued with their unlawful activities”.
Newsday reports that one worker shouted at her:
“You no longer have any power to remove us. This is the new dispensation, we do what we want”.
Last year Mrs Mugabe was widely reported to be preparing to replace her ageing husband as president of Zimbabwe.
But the rumours were not welcomed by parts of the ruling party and he was ousted from office in November, to be replaced by his long-time ally Emmerson Mnangagwa.
Source: BBC News