Officers were called to four homes in East Lancashire on Saturday evening, after reports that crowds of people were seen attending house parties in breach of coronavirus restrictions.
Lancashire Police said three of the illegal parties were held at homes in Blackburn and another in the district of Pendle.
Detective Chief Constable Terry Woods took to Twitter to express his disappointment at the continued flouting of restrictions.
He said: "Disappointingly we saw an increase in COVID incidents this weekend, the biggest issue being house parties - 31 fines issued at 4 house parties in East Lancashire (3 Blackburn, 1 Pendle).
"Please let’s not drop our guard against COVID."
The whole of Lancashire is currently under tier 3 restrictions, which means we are currently prohibited from mixing with different households to help reduce the risk of transmission.
Responding to reports of the illegal house parties, Dr Sakthi Karunanithi, director of public health for Lancashire, said "now is not the time to let our guards down".
He said: "Overall we are seeing a positive trend in Lancashire since October. Now is not the time to let our guards down.
"Please avoid household mixing to help disrupt the spread of the virus."
The tier system will be reviewed on Wednesday (December 16), but it is expected that Lancashire will remain under the tightest restrictions until after the New Year.
The latest data from Lancashire Police suggests that more than 800 fines have been handed out to people across the county for breaching coronavirus restrictions, including attending large gatherings, non-essential travel and refusal to wear face masks in shops, venues or public transport.
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