- Video report by ITV News Security Editor Rohit Kachroo
Theresa May has accused Russia of meddling in international elections and spreading fake news.
The Prime Minister hit out at Moscow for attempting to "sow discord in the West and undermine our institutions" and she accused the state of trying to "weaponise information".
She said: "I have a very simple message for Russia. We know what you are doing. And you will not succeed."
But she also stressed for the need to engage with Russia to avoid a return to the Cold War.
Russia has rejected the accusations as "irresponsible and unfounded", suggesting that the Prime Minister is trying to deflect attention from concerns over domestic politics.
Addressing the Lord Mayor's Banquet at the City of London's Guildhall, Mrs May stressed Britain's "shared commitment to open economies and free societies in the face of those who seek to undermine them.
"Chief among those today, of course, is Russia."
She said Russia has "mounted a sustained campaign of cyber espionage and disruption" in her speech.
"This has included meddling in elections, and hacking the Danish Ministry of Defence and the Bundestag, among many others.
"It is seeking to weaponise information. 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.
"So I have a very simple message for Russia.
"We know what you are doing. And you will not succeed. Because you underestimate the resilience of our democracies, the enduring attraction of free and open societies, and the commitment of Western nations to the alliances that bind us.
"The UK will do what is necessary to protect ourselves, and work with our allies to do likewise."
Mrs May also highlighted Russia's annexation of Crimea as the first time since the Second World War that one sovereign nation has forcibly taken territory from another in Europe.
She evoked the hope that greeted the fall of the Soviet Union, saying Britain and Russia should not be "in a state of perpetual confrontation", and that Foreign Secretary Boris Johnson will visit Moscow in the coming months for talks.
"We will take the necessary actions to counter Russian activity," the PM will say.
"But this is not where we want to be - and not the relationship with Russia we want.
"We do not want to return to the Cold War, or to be in a state of perpetual confrontation.
"So whilst we must beware, we also want to engage - which is why in the coming months the Foreign Secretary will be visiting Moscow.
"For there is another way.
She expressed hope that Moscow may choose a "different path" in which it works with allies to promote international stability.
But she warned "For as long as Russia does not, we will act together to protect our interests and the international order on which they depend."
Russian Foreign Ministry spokeswoman Maria Zakharova responded Tuesday, calling May's statements "irresponsible and unfounded."
She said that a pledge to uphold the international order sounds particularly "cynical" given British action in Iraq and Libya that "caused suffering of millions and destabilized entire regions."
Ms Zakharova argued that Britain is facing hard times following its decision to leave the European Union, adding that "Russia has been chosen as a foreign enemy needed to deflect public attention."