“There is strong evidence that these elections fell short of SADC’s own guidelines and the Zimbabwean electoral law,” he said in a statement.
“We are concerned about the potential implications for the region.”
Mr Mugabe, 89, ushers in a new five-year term in a 60,000-capacity stadium in a ceremony to which around 40 leaders have been invited. Organisers say the event will be similar to Mr Mugabe’s inauguration as prime minister in 1980.
Mr Tsvangirai will not attend the inauguration.
“Expecting Tsvangirai to attend the inauguration is like expecting a victim of robbery to attend a party hosted by the robber,” Tsvangirai’s spokesman Luke Tamborinyoka said.
Edited by Chris Irvine