Stating that Mugabe's government had no legitimacy, he called for a transitional government to reflect the results of the March 29 presidential elections, which was won by the opposition.
Calling for international sanctions against Mugabe, Miliband urged South Africa and the rest of the international community to "unite behind a tough, strong, clear [UN] security council resolution".
Mugabe claimed victory in June's presidential election run-off after his opponent, Morgan Tsavangirai, who heads the Movement for Democratic Change, pulled out of the race following widespread reports of violence and intimidation.
|Millions of Zimbabweans have fled the country [GALLO/GETTY]
But back in London, refugee groups criticised the British government for threatening to send 11,000 refugees back to Zimbabwe after refusing their applications for asylum, cutting off financial help and denying them jobs.
Al Jazeera's Richard Bestic, reported from London that the British interior ministry had said it would not force failed asylum-seekers to return to Zimbabwe.
But refugee groups say letters were sent to hundreds of Zimbabwean asylum-seekers asking them to leave the UK.
Zimbabwe's political conflict has forced thousands to seek refuge in neighbouring African nations or at foreign embassies in the capital, Harare.