More than a dozen attacks have been linked to a string of terrorist incidents in Britain this year, including one involving a suspected Islamist attack on a steel factory.
The attacks have also been linked by intelligence officials to the attacks in France and Germany.
The police and intelligence agencies have also said they are looking at possible links to a plot in the United States, where the FBI has said it is investigating the possibility of a “lone wolf” attack.
“We have a number of reports of people who have been arrested,” said Chief Superintendent Mick Smith of the Metropolitan Police.
“There have been multiple arrests for individuals in relation to this incident.”
He said there were “no indications” of any involvement by an individual in the attacks.
The FBI, Britain’s national security agency, said it was working with the US Department of Homeland Security (DHS) to “assess the nature of the threat”.
“We remain in close contact with our UK counterparts on the threat from international terrorism,” the FBI said in a statement.
“At this time, it is not possible to comment on the specifics of the investigation or the individuals involved.”
UK Prime Minister Theresa May was criticised for a statement in April saying the “real threat” of terrorism was not coming from the UK but from the Islamic State group (IS).
But a separate report by the Home Office said the “threat remains real” in Britain and was likely to grow, including from “lifestyle extremism” such as “radicalisation by social media”.
It also said there had been an increase in people travelling from the United Kingdom to join IS.
In May, the Prime Minister also announced an end to the country’s three-year-old ban on returning foreign fighters returning to the UK.
She said the ban had been in place for more than a year but was now “not necessary”.
“In addition, it means we will no longer be able to bring people back from a safe country into the UK, where they could be radicalised, radicalised again and potentially join terrorist organisations.””
It’s vital we work together to make sure this threat is managed effectively.” “
In addition, it means we will no longer be able to bring people back from a safe country into the UK, where they could be radicalised, radicalised again and potentially join terrorist organisations.”
It’s vital we work together to make sure this threat is managed effectively.
“The report from the Home Affairs Select Committee said the Government had failed to stop IS militants returning to Britain from the Middle East.
It also accused ministers of ignoring the threat, saying that “they have failed to do their job to deal with the root causes of the growing threat to our country”.
The Home Office has also been criticised for failing to protect the public.
It has not responded to a request for comment.