Police have launched a criminal inquiry into allegations of anti-Semitic hate crimes within the Labour Party.
Met Police chief Cressida Dick told the BBC her officers were assessing online material because it appears "there may have been a crime committed".
It comes after LBC Radio obtained what it said was an internal Labour document detailing 45 cases, involving messages posted by members on social media.
Labour MP Margaret Hodge said the party should have notified the police itself.
"I think they have a duty of care to their members where there are allegations of race hate crimes," she told BBC News.
"For me as a Jewish Labour MP... there is a feeling of fear about and that is why firm action taken by the police and the party is so important."
LBC handed the dossier to Ms Dick in an interview in September.
Before it did so, it showed the material to former Met police officer Mak Chishty, whose view was that 17 instances should have been reported to the police for investigation, and another four were potential race hate crimes.
LBC said the allegations included threats against members of Parliament, including a message on Facebook that a female Labour MP was a "zionist extremist...who hates civilised people" and is "about to get a good kicking".
'Full force of law'
The Met Commissioner told BBC Radio 4's Today programme her officers were seeking advice from the Crown Prosecution Service and hoped to conclude the investigations quickly.
She made it clear the Labour Party itself was not under investigation.
"We would always want institutions and political parties and similar to be able to regulate themselves.
"However, if somebody passes us material which they say amounts to a crime we have a duty to look at that and not just dismiss it.
"We have been assessing some material that was passed to me, in a radio studio of all things, about two months ago and we are now investigating some of that material because it appears there may have been crime committed."
It is understood Labour has not been contacted by the police and has not been told of the exact nature of the allegations being investigated.
A party spokesman said a "robust system" was in place for investigating complaints of alleged breaches of Labour rules by its members.
"Where someone feels they have been a victim of crime, they should report it to the police in the usual way."
Deputy Labour leader Tom Watson told Today he was "not surprised" by the allegations and anyone engaging in such behaviour must have the "full force of the law" applied to them.
"If this does one thing, it will silence a small number of people, who still believe that anti-Semitism doesn't exist in my party or in other parties, and that hampers the campaign to try and deal with this problem as quickly and as swiftly and as forcefully as we can," he said.
In September, Labour's ruling body agreed to adopt in full an international definition of anti-Semitism, after a long-running row about Jeremy Corbyn's handling of the issue.
It vowed to incorporate all the 11 examples of anti-Semitism cited by the International Holocaust Remembrance Alliance into its code of conduct. But Jewish groups criticised an accompanying statement which the party said was aimed at protecting free speech.
The party is currently working through a backlog of cases of alleged anti-Semitism complaints against members.
But MP Dame Louise Ellman, vice-chair of Labour Friends of Israel, suggested the party had failed to properly investigate "potentially very serious" cases and she hoped, at the very least, Mr Corbyn was aware of the file.
"The disgraceful position Labour is in is continuing," she told BBC News. "I would have thought the people responsible would have learnt their lesson by now. It appears they have not done so."
The Board of Deputies of British Jews said there was a "deeply embedded" culture of anti-Semitism within parts of Labour.
"This comes as no surprise to us," said the organisation's vice president Amanda Bowman.
"We have repeatedly set out what Labour needs to do, including taking firm action against anti-Semites and making its opaque processes transparent."