Three terror attacks have been prevented in the UK since the coronavirus pandemic began, according to security services.
Arrests for terrorism-related activity fell by 34% in 2020, the lowest level in nine years, Home Office data showed.
But the number of terror plots stopped has risen to 28 since March 2017, counter terror police said.
The foreign secretary said young people had been recruited to terror groups in "worrying" numbers during lockdown.
Dominic Raab told the House of Commons there had been a recent rise in the volume of terror-related content online.
"We can see a worrying rise in the proportion of children and teenagers that are now being arrested for terrorism offences," he added.
The UK's senior national coordinator for Counter Terrorism Policing warned that these organisations "have not stopped planning attacks or radicalising vulnerable people online".
Deputy Assistant Commissioner Dean Haydon said officers had made 185 arrests across more than 800 live investigations during the pandemic - "stopping three possible terror attacks in the process".
He said as lockdown eases, "there will be greater opportunity for terrorists to operate".
"We want the public to join the police, security staff and retail workers in a collective community effort to minimise the chance of attack" he added.
In February, the UK's terrorism threat level was downgraded from " severe" to "substantial" , which means an attack is still likely.
Mr Haydon also warned of a "worrying trend" of children under the age of 18 being arrested for terrorism-related activities.
This age group was the only one that saw an increase - rising from 11 to 17 in total, Home Office data showed.
"That is a total of 10% of all arrests, which has risen from just 4% in the previous 12 months," said Mr Haydon.
Police have launched a website and helpline to offer advice and support for families to prevent young people becoming involved in terrorism, he said.