Brown said a food crisis and cholera epidemic in Zimbabwe which have killed hundreds of people was an "international rather than a national emergency" that demanded a coordinated response.
"We must stand together to defend human rights and democracy, to say firmly to Mugabe that enough is enough."
While Brown did not explicitly call for Mugabe to step down, his comments echo calls on Friday from United States Secretary of State Condoleezza Rice, who said Mugabe’s departure from office was long overdue.
Britain, the former colonial power, has long had particularly tense ties with Zimbabwe, and Mugabe has accused London of being reluctant to give up its influence over the African nation.
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 also said he hoped the United Nations Security Council would meet "urgently" to consider the situation in Zimbabwe.
"The people of Zimbabwe voted for a better future," he said, "It is our duty to support that aspiration."
Zimbabwe’s neighbours in the 15-nation Southern African Development Community have so far failed to persuade Mugabe and the opposition to form a unity government.
But, faced with Zimbabwe’s worsening economic collapse and the humanitarian crisis spilling over into their own countries, they may now be forced to take a stronger stand against the veteran Zimbabwean leader.
Brown said the immediate priority was to prevent more deaths by distributing rehydration packs and medical testing kits. He said a "command and control structure" should be put in place in the Zimbabwean capital Harare to coordinate aid efforts.