With results from all but four of the 31 voting districts counted by early Monday, the ruling party had won 21 seats. The opposition Movement for Democratic Change took six, state radio reported.
Results from four remote districts in western Zimbabwe had been delayed by heavy rains, preventing returns from reaching regional centres, the radio said.
The results showed that the ruling Zimbabwe African National Union-Patriotic Front (Zanu-PF) would control at least 56 of the 66 seats in the Senate, which was established by a constitutional amendment pushed through by Mugabe this year.
Ruling party candidates were unopposed for 19 of the Senate's 50 elected seats. Mugabe appoints six other seats, and 10 are reserved for traditional leaders, selected by the pro-government Council of Chiefs.
Saturday's election was marked by a record low turnout blamed on voter apathy and the deep divisions in the opposition over boycott calls.
Independent monitors predicted that an overall average turnout for the 31 contested seats across the country was between 15% and 20%, the lowest in any national poll since independence in 1980.
Opposition leader Morgan
Tsvangirai boycotted the poll
Morgan Tsvangirai, leader of the MDC, thanked supporters on Sunday 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 the MDC divided, threatening to destroy the only group to have seriously challenged Mugabe's increasingly autocratic 25-year rule.
The rift, prompted by Tsvangirai's call for a boycott of Saturday's poll, deepened on Sunday as Tsvangirai rejected attempts to suspend him from the party's leadership.
Tsvangirai said in a statement that Gibson Sibanda, his deputy, was in breach of the party's constitution by notifying him that the party's disciplinary committee had suspended him from the organisation's presidency.
He said that only the party's annual convention, scheduled for February, could remove him.
The suspension "reflects the desperate and immature state of mind gripping my erstwhile comrades", Tsvangirai said.
According to Sibanda, the disciplinary committee barred Tsvangirai with immediate effect from party offices and demanded that he return all party materials and property but said he could keep two party cars for his personal use.
Tsvangirai argued that participation in Saturday's vote would lend credibility to a poll that was certain to be flawed. But Sibanda and senior members of his party, mostly from Bulawayo and the western Matabeleland province, rejected his boycott call and fielded 26 candidates countrywide.
Voters gather at a polling station
north of the capital Harare
In preliminary results, Sibanda's group won five Senate seats in his home town, the second city of Bulawayo, and one outside Bulawayo.
Voter turnout in Bulawayo was 6% of eligible electors, the independent Zimbabwe Election Support Network said.
Across the country, "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.