Nelson Chamisa, spokesman for Movement for Democratic Change (MDC), which agreed a power-sharing deal with Mugabe in September, said that the swearing in of the two vice-presidents was "meaningless".
"Whatever appointments or acts that do not address the woes of the country are meaningless," he said. "Any appointments by Mr Mugabe that do not take the country out of the current economic quagmire are meaningless."
Morgan Tsvangirai, the MDC leader, on Sunday threatened to pull out of the power-sharing agreement, which has been stalled as the two sides have been unable to agree on the allocation of ministry posts.
Tsvangirai accused Mugabe of trying to secure all important ministries for Zanu-PF and said that he was willing to continue negotiating only if there was a chance of a breakthrough.
His warning followed the news on Saturday that Mugabe had allocated significant ministries, such as the ministry of defence and home affairs, to his own party.
Citing a government gazette, a list published in The Herald newspaper gave Mugabe's party 14 ministries including the portfolios of defence, home and foreign affairs and justice as well as local government and media.
Tsvangirai has reportedly argued that if the defence ministry post goes to Zanu-PF, then the home affairs post, which covers policing, must go to the MDC.
Tsvangirai said he was prepared to renegotiate if Mugabe followed through with his stated allocation of ministries.
"That is not power sharing, it is power grabbing," he told a rally.
"The people have suffered. But if it means suffering the more in order for them to get what is at stake, then so be it."
Mbeki is travelling back to Harare in an attempt to break the impasse, his spokesman confirmed.
Under the agreement he brokered, Mugabe remains the president while Tsvangirai takes the newly created post of prime minister.
Tsvangirai's MDC has been allocated 13 ministries, among them constitutional and parliamentary affairs, economic planning and investment promotion, labour and social welfare, sport, arts and culture, and science and technology development.