"This is the 11th time that Tsvangirai has threatened to withdraw from the presidential run-off and on each occasion I have challenged him to put it in writing as required by the law," he said.
Mr Tsvangirai's announcement of withdrawal was to avoid "humiliation", he added.
"Tsvangirai went into the election thinking that it was a sprint and was not prepared for a marathon and wants to avoid defeat," he said.
"He spent his time globe-trotting and gallivanting in Europe and left MDC-T supporters without leadership."
There has been no response from Mr Mugabe himself to Mr Tsvangirai's announcement.
On Sunday, Mr Tsvangirai said that there was no point running when elections would not be free and fair and "the outcome is determined by... Mugabe himself".
He said that while the decision had been a difficult one it was necessary to protect the people of Zimbabwe.
The opposition's decision was announced after its supporters, heading to a rally in the capital Harare, came under attack.
The BBC's Peter Biles says Mr Tsvangirai did not want to expose his supporters to any more violence.
MDC stands for Movement for Democratic Change. If the only recourse for the people to change a regime is armed conflict - the next regime will be no better than the last. Matabele, Bulawayo, Zimbabwe
He will now be hoping that the Sadc will refuse to confer legitimacy on the process, he says.
Our correspondent adds that the country's economic crisis - with unofficial figures putting inflation at 2m% - could drive Zimbabwe's government to negotiate for a political solution.
The MDC won the parliamentary vote in March, and claims to have won the first round of the presidential contest outright.
In the official results, Mr Tsvangirai led but failed to gain enough votes to avoid a run-off.
