The by-election in the central town of Kadoma pitted the ruling Zimbabwe African National Union - Patriotic Front (Zanu-PF) against the opposition Movement for Democratic Change (MDC).
Voting was reported to be slow during the two-day poll, but the official Electoral Supervisory Commission (ESC) was optimistic of a respectable turnout by the close of voting.
"In the morning the numbers were, in some polling stations, higher than expected," ESC spokesman Thomas Bvuma said. He said people were still voting by mid-afternoon.
State radio reported that 12 821 out of more than 45 000 registered voters had cast their votes by mid-morning yesterday.
The peacefulness of Sunday's voting contrasted with the first day of the by-election when the opposition claimed its supporters were intimidated by ruling party loyalists.
"It's certainly quieter than yesterday (Saturday)," MDC candidate Charles Mpandawana said by telephone from the town, some 140 km southwest of Harare.
"It paints a better picture".
The opposition party claimed that veterans of Zimbabwe's 1970s liberation war blocked off a polling station and fired their weapons to scare away opposition voters.
This was denied by the electoral commission's spokesperson.
"There was no violence or shooting," Bvuma told AFP on Sunday. He said the poll had been "calm and peaceful".
Mpandawana is contesting the election to fill the post left vacant by the death in August of his father, Austin Mpandawana, also of the MDC.
But Tichafa Mutema of Zanu-PF is looking to wrest the seat away from the opposition party.
Yesterday the state Zimbabwe Broadcasting Corporation reported that the ruling party's election manager was predicting a "landslide victory" for Mutema.
Ballot counting is scheduled for today, with a result expected later.
The Kadoma by-election comes at a crucial time for Zimbabwe, with a Commonwealth summit due to be held from Wednesday in the Nigerian capital Abuja likely to deliberate on Harare's readmittance to the councils of the 54-member grouping.
The southern African country was suspended from the Commonwealth councils in March last year after some international observers said a presidential election that returned Mugabe to power was marred by violence, intimidation and electoral flaws.
Mugabe rejects the charges and accuses white member nations of the group, especially Britain and Australia, of pursuing a racial vendetta against Zimbabwe because of a controversial land reform programme.
The three-year old programme has seen thousands of white-owned farms in Zimbabwe seized for redistribution to new black farmers.
On Friday the 79-year-old leader threatened that Zimbabwe would have to "say goodbye" to the Commonwealth if it was not given equal status and called for solidarity to be shown by other African nations.
Commonwealth Secretary General yesterday said the issue was unlikely to open a chasm between white and African members. But he did acknowledge a "very wide variety of views" among African countries over how to deal with their southern African compatriots.
McKinnon has previously said the Commonwealth has laid down "markers" for Zimbabwe to achieve before it can be readmitted to the Commonwealth, which include ending harassment of opposition members and involving the United Nations in the land reform programme. – Sapa-AFP.