The UK's Counter Terrorism Policing chief Laurence Taylor has warned his officers and the security services are busier than they have been for many years.
The Scotland Yard Assistant Commissioner said that the threat from state actors, like Iran and Russia, had increased by 50 per cent over the past year and showed no sign of slowing down.
AC Taylor told a briefing of security journalists: "It does feel like our caseload is like juggling.
"We're keeping many high-priority balls in the air simultaneously, each of them requiring careful attention and monitoring.
"There's no let up to that. The big difference for us is, we can't afford to drop one of those balls.
"In just the last two weeks, we have been facing down an incredibly complex threat picture.
"We're seeing fundraising for terrorism purposes. We're seeing overseas travel or certainly intent to travel overseas with terrorism intent.
"We're seeing increased ambition from Islamist groups, to lone individuals accessing extreme right-wing terrorism content online."
In a stark analysis of the threat facing the UK, the senior officer warned: "It feels very much like we're on a knife-edge of extremism and polarising language that's cutting through society, and that is leading to our doorstep in counter-terrorism policing.
“To be really clear, we do not and will not tolerate those involved in terrorism in any form, or working on behalf of a foreign state, whether they know it or not.
"And we will do all we can to stop them and bring them to justice."
The briefing comes a day after the Home Office introduced new legislation bringing stiffer penalties for those acting on behalf of hostile states, with sentences of up to 14 years.
Counterterrorism still remains the largest proportion of the caseload facing terror police.
In April, the terror threat level was raised from 'Substantial' to 'Severe', meaning an attack is "highly likely".
It followed last October’s fatal terror attack on a Synagogue in Manchester and a stabbing attack and firebombings targeting the Jewish community in north London.
Police say much of the increase in terrorism has been driven by global conflicts.
Laurence Taylor said: “What plays out abroad will always have a link back to our shores and the type of threats that we are dealing with.
“And we're seeing that in global instability now.
"And probably one of the clearest examples of this has been the rise in anti-Semitism, an anti-Semitic incident on our streets."
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