About 2,000 crimes related to online abuse are being reported to the police in London each year, according to new figures.
The Met Police said the number of incidents related to online harassment and bullying had increased by 3.1% compared to the previous year.
Deputy Commissioner Craig Mackey told members of the London Assembly the problem could get worse.
Mr Mackey said: "I think it will rise as people become more aware of this."
The admission follows a number of high-profile cases of "Twitter trolling", where users of the social networking site were bombarded with threats and abuse.
The police said investigators faced difficulties as many social media companies are based in the US, where laws are different.
Mr Mackey also revealed that trolls often use fake identities or software to cover their IP addresses, which can lead to difficulties in obtaining evidence for court cases.
Earlier this year Caroline Criado-Perez faced abuse and threats of rape after successfully campaigning for a woman to be featured on UK banknotes.
She complained it was "appalling" she had to trawl through the abuse to find evidence for the police.
"I got an email saying 'why don't you go through what you've had' from three users with certain handles," said Ms Criado-Perez.
"It just shows such insensitivity to someone who's been through so much."
On Thursday, Mr Mackey said the force had investigators who could have looked through the material.
He said: "I don't know why she went back through each and every one."
Police are also investigating a series of sexually-explicit and violent tweets sent to Labour MP Stella Creasy.
Television historian Professor Mary Beard and newspaper columnist Hadley Freeman have also faced abuse online.
Twitter has recently introduced a "report Tweet" button to try and combat trolling.