In a statement, Mr Brown said that over the coming days the first priority would be delivering aid to Zimbabwe, such as rehydration and testing packs.
Gordon Brown: 'The states around Zimbabwe are getting increasingly angry'
He said: "This is now an international rather than a national emergency. International because disease crosses borders.
"International because the systems of government in Zimbabwe are now broken. There is no state capable or willing of protecting its people.
"International because - not least in the week of the 60th anniversary of the universal declaration of human rights - we must stand together to defend human rights and democracy, to say firmly to Mugabe that enough is enough."
Mr Brown said he had "been in close contact with African leaders to press for stronger action to give the Zimbabwean people the government they deserve".
He added: "The people of Zimbabwe voted for a better future. It is our duty to support that aspiration."
He also said he hoped the United Nations Security Council would meet "urgently" to consider the country's plight.
Liberal Democrat leader Nick Clegg said it was time for the UN to support the "justified" use of military force as a means for the international community to protect Zimbabweans.
"The world has sat idly by whilst Robert Mugabe has brutalised his own people for too long.
"Economic recession in the West has led the world to avert its gaze from the suffering in Zimbabwe.
"The UN must urgently declare that Mugabe will be indicted in the International Criminal Court."
He said it would be "inexcusable" if no further action was taken soon.
Tutu: 'He's destroyed a wonderful country'
He called on China to stop blocking international action through its veto on the Security Council, and South Africa to take a "tougher" stance with its neighbour.
Labour MEP Glenys Kinnock told BBC Radio 4's PM programme sanctions could have a limited impact but the threat of cholera might inspire other African leaders to act.
She said a great deal of diplomacy was taking place but she did not know how much effect this would have.
"I think efforts are being made to talk with those who are likely to influence Mugabe.
"But the reality is that he is impossible to influence.
"He is so manipulative and so devious that influencing him is very difficult indeed," she said.
They have joined a growing list of international leaders in condemning Mr Mugabe.
US Secretary of State, Condoleezza Rice, said it was "well past time" for him to leave office.
And the Nobel Peace Prize laureate, Archbishop Desmond Tutu, said African nations should come together to use military force if Mr Mugabe refused to go.
Archbishop Tutu said Mr Mugabe had committed "gross violations" against Zimbabwe's people and ruined "a wonderful country".
His comments came a day after Kenya's Prime Minister Raila Odinga said African governments should oust Zimbabwe's leader.
Mr Mugabe and opposition leader Morgan Tsvangirai agreed to share power in September to tackle the country's economic meltdown but they have been unable to agree on the allocation of cabinet posts.
The deadlocked agreement followed disputed elections, which both men claimed to have won.
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