Terror-related crime 'stretches police' - Scotland Yard
Police are being "stretched" by an "exceptionally high" number of inquiries into Islamist-related extremism, Scotland Yard has said.
Assistant Commissioner Mark Rowley also said "dozens" of UK families had sought police help in stopping relatives travelling to Syria.
He spoke of a "step-change" in counter-terrorism as foreign conflicts such as the one in Syria attract UK nationals.
Mr Rowley said the challenge for police would "run for years".
There have been 218 terrorism-related arrests in the England, Scotland and Wales this year, with 16 people charged with alleged extremist activity in Syria.
Scotland Yard said the volume, range and pace of counter-terrorism activity had changed dramatically as conflicts in Syria and other countries drew in people from the UK.
Mr Rowley - Scotland Yard head of counter-terrorism - added that the threat featured a "new dimension", amid growing fears over the influence of Islamic State (IS).
He said officers were foiling several attack plots each year, including what he describes as "complex conspiracies" and "spontaneous yet deadly attacks".
He added that in some cases police had acted early, even if that meant there was no prosecution.
"There are some different dynamics today, where we have an increasing number of people who weren't previously on the terrorism radar being attracted by an ideology they see on social media," he said.
Counter-terrorism police chiefs have previously appealed to Muslim women to persuade relatives not to go to Syria to fight - about 500 Britons are known to have travelled there to get involved in the conflict.
That campaign prompted campaigners to question if the police were trusted to get their message across.
However, Mr Rowley said "dozens" of families had contacted officers this year with concerns that their sons or daughters could have been radicalised.
"We had a case in the last few days where a young person was travelling out to Syria," he said.
"The family realised very quickly, reported it to us so quickly we were able to get them intercepted when they got into Turkey and the Turks turned them back to the UK."
Dominic Casciani, BBC home affairs correspondent
The arrest figures from Scotland Yard show this is shaping up to be one of the busiest years for counter-terrorism investigations since 9/11 - and the numbers have been creeping up ever since foreign fighters began heading to the Syrian civil war.
Police know that arrests alone won't solve the problem: not every arrest will ultimately lead to a charge - and some of those cases may never stand up to scrutiny in court.
The security services can stop people at airports, seize cash and try to monitor and disrupt suspected extremist activity at its earliest stage.
But, in practical terms, they can't stop all the people all of the time. And that's why the support and co-operation of families, who fear for the safety of sons or daughters, is a critical tool.
Those families are now the early-warning system in the battle against IS-related extremism in the UK.
Mr Rowley said police were also focused on finding the "recruiting sergeants, funders and advocates" within the UK, as well as battling the influence of extremist videos online.
Since August, IS has killed four Western hostages - US journalists James Foley and Steven Sotloff and British aid workers David Haines and Alan Henning - releasing videos of their apparent beheading.
Mr Rowley said counter-terrorism officers were removing more than 1,000 online postings a week, including graphic and violent videos and images.
"Of course we can't clean it all up, but it is very effective."
Mr Rowley said "many" Britons known to have travelled to Syria had now returned to the UK and that police were attempting to prosecute them.
'Run for years'
He said police were aware some had already died fighting in the Middle East, but warned that those who do return home posed "a real threat" to the UK.
"This is a terrorist threat that may have its seat somewhere that seems a long way away but it is influencing people on the streets of London and is trying to import terrorism onto the streets of London and the UK," he added.
"I think we are seeing a step change that is going to run for years.
"None of us have a crystal ball but looking at the situation we are seeing in Iraq and Syria, despite the best efforts of governments across the world, this looks like a step change that we are going to be wrestling with for some time to come."