People are violating lockdown rules as well as setting fires while they are out, according to two north-east of England fire services.
Tyne and Wear Fire Service (TWFRS) said there had been an "extreme increase" in rubbish and grassland fires.
Cleveland Fire Brigade said it was also tackling an increasing number of deliberate fires.
Chief Fire Officer Chris Lowther said it was an area of "great concern", and urged people to stay at home.
"It worries me greatly that people are not only ignoring the mandate to stay at home to protect the NHS but are also placing additional strain on firefighters," he said.
"People need to ensure their behaviour is in line with what the public expect and parents should know where their kids are. Lockdown means lockdown."
TWFRS said it normally saw a spike in outdoor - so-called secondary - fires during lighter nights.
But on Monday it received 98 calls and attended 47 cases, most of them deliberate, which was a 20 per cent increase on what the service would normally expect.
The service has not seen an rise in more serious primary fires despite people spending more time at home, the Local Democracy Reporting Service said.
Cleveland Fire Brigade said it had a "zero-tolerance" approach to arson and would prosecute where necessary.
Three deliberate fires at Summerhill Country Park in Hartlepool at the weekend prompted concerns that young people were involved.