Russia has attempted to attack the UK's media, telecommunications and energy sectors, an intelligence chief has said.
Ciaran Martin, the chief executive of the National Cyber Security Centre, a part of GCHQ, said the UK is facing "two significant groups of threats in cyberspace": hostile states and rampant criminality.
"The Prime Minister sent Russia a clear message on Monday night – we know what you are doing, and you will not succeed," Mr Martin said at a meeting held in London on Wednesday.
Theresa May accused Russia of "seeking to weaponise information" as she accused Moscow of meddling in elections and hacking Denmark's defence ministry and the German parliament.
She also accused Moscow of "deploying its state-run media organisations to plant fake stories and photo-shopped images in an attempt to sow discord in the West and undermine our institutions".
NCSC was set up a year ago as the public-facing and defence-focused side of the UK's signals intelligence agency GCHQ.
In that time it has responded to more than 600 significant incidents, some of which related to hostile states, and others from what Mr Martin called "rampant criminality in cyberspace".
The NCSC chief said there were basic things that could be done to protect against both threats, although he said nation-state attacks could be very sophisticated.
Mr Martin said he couldn't "get into too much of the details of intelligence matters, but I can confirm that Russian interference… has included attacks on the UK media, telecommunications and energy sectors.
"That is clearly a cause for concern – Russia is seeking to undermine the international system. That much is clear. The PM made the point on Monday night – that international order as we know it is in danger of being eroded."
Russia has denied claims of meddling in elections, including last year's US presidential vote.