Mugabe extended his rule as president after standing as the only candidate in a June 27 election boycotted by Morgan Tsvangirai, leader of the opposition Movement for Democratic Change (MDC).
Tsvangirai cited a campaign of violence and intimidation against his supporters by loyalists of Zanu-PF, Mugabe's party, as the reason he quit the election.
The G8 leaders called on Mugabe's government to "work with the opposition", adding that any mediation effort should respect the results of Zimbabwe's first round presidential election on March 29, which Tsvangirai won.
They also recommended the appointment of a special UN envoy to report on Zimbabwe's humanitarian and human rights situation.
The group statement comes days after the US introduced a UN Security Council resolution that would impose new sanctions against Mugabe and his closest allies in Zimbabwe.
Nicolas Sarkozy, France's president, said on Monday that France was prepared to endorse the resolution, while Angela Merkel, the German chancellor, has backed the US move.
A senior Russian official said on Tuesday that Moscow did not support the sanctions.