Partial results from Sunday’s vote showed the ruling Zimbabwe African National Union-Patriotic Front would control at least 49 out of 66 seats in the Senate, which was established by a constitutional amendment pushed through by Mr Mugabe earlier this year.
The poll was marked by a record low turnout blamed on voter apathy and the deep divisions in the opposition over boycott calls.
Independent monitors predicted an overall average turnout for the 31 contested seats across the country of between 15 and 20 per cent, the lowest in any national poll since independence in 1980.
Opposition leader Morgan Tsvangirai has thanked supporters for “heeding our call for the boycott of this meaningless election.”
“We have been vindicated. We were proved right in our assessment of the national sentiment,” he said, referring to his boycott campaign.
However, the election left his Movement for Democratic Change bitterly divided, threatening to destroy the only group to have seriously challenged Mugabe’s increasingly autocratic 25-year rule.
The rift, prompted by Mr Tsvangirai’s call for a boycott of the poll, deepened as Mr Tsvangirai rejected attempts to suspend him from the party’s leadership.
Mr Tsvangirai said in a statement his deputy, Gibson Sibanda, was in breach of the party’s constitution by notifying him the party’s disciplinary committee had suspended him from the organisation’s presidency.
He said only the party’s annual convention, scheduled in February, could remove him.
The suspension “reflects the desperate and immature state of mind gripping my erstwhile comrades”, Mr Tsvangirai said.
According to Mr Sibanda, the disciplinary committee barred Mr Tsvangirai with immediate effect from party offices and demanded he return all party materials and property but said he could keep two party cars for his personal use.
Mr Tsvangirai argued that participation in the vote would lend credibility to a poll that was certain to be flawed.
But Mr Sibanda and senior members of his party, mostly from Bulawayo and the western Matabeleland province, rejected his boycott call and fielded 26 candidates countrywide.
In preliminary results, Mr Sibanda’s group won five Senate seats in his hometown, the second city of Bulawayo.
But voter turnout in Bulawayo was just 6 per cent of eligible electors, the independent Zimbabwe Election Support Network said.
“Turnout was very low indeed. It is a vote of no confidence in the entire process,” said Reginald Matchaba-Hove, head of the election support network.
Nearly 60 per cent of registered voters cast their ballots in the last parliamentary polls in March.
Results for 19 out of 31 contested seats showed the ruling ZANU-PF gained control of 14, with five in Bulawayo going to the opposition.
ZANU-PF won all three seats in Harare, traditionally sympathetic to the opposition.
The ruling party’s candidates were unopposed for 19 of the Senate’s seats.
Mr Mugabe appoints six other seats, and 10 are reserved for traditional leaders, selected by the fiercely pro-government Council of Chiefs.
Mr Mugabe abolished the upper chamber in 1990 but decided to recreate it in what critics said was an attempt to increase his power to dole out jobs and perks as the country faces its worst economic crisis since independence in 1980.
The new house has no veto powers over legislation passed by the lower house, which is dominated by the ruling party.