Youths went on the rampage on Thursday in northern Zambia's Copper Belt, a stronghold of opposition leader Michael Sata who is leading the race to become the next president of Africa's biggest copper producer.
The latest tally from Tuesday's election showed Sata with 639,787 votes against 542,362 for incumbent Rupiah Banda, leader of the Movement for Multi-party Democracy (MMD) party that has run the former British colony since one-party rule ended in 1991.
The Election Commission tally was based on 85 out of 150 constituencies.
Despite the strong showing for 74-year-old Sata -- nicknamed King Cobra for his vicious tongue -- the drip-by-drip release of results has sparked rumours of vote rigging, angering Sata supporters in particular.
Hackers who hit the Election Commission website overnight, posting false results showing Sata on course for a landslide, have only added to the confusion and tension of what was already a neck-and-neck run-off between two old rivals.
Youths fought running battles with riot police in the northern Copper Belt towns of Ndola and Kitwe, 250 km (150 miles) north of Lusaka, setting fire to vehicles and markets in the normally peaceful southern African country's economic heartland, police, residents and local media said.
They are on the streets with stones and we can only urge them to stop the riotous behaviour, Copper Belt police chief Martin Malama told Reuters. There were no reports of injuries and it was too early to assess the extent of damage, he added.
State radio reported that crowds of protesters had also blocked the main road between Kitwe and Chingola, another mining town near the border with Democratic Republic of Congo, suggesting high levels of tension across the region.
Despite the unrest, Election Commission Chairwoman Irene Mambilima said she would not be rushed into releasing results that had not been double- and triple-checked.
The verification of these results is very important. We ask political parties to tell their supporters to remain peaceful, she told reporters, adding that the full result should be known by Thursday evening.
The High Court in Lusaka earlier on Thursday banned three private media outlets from speculating on the outcome of the September 20 election, further angering Sata's young urban support base.
The European Union's observer mission said the poll had been generally well organised and that ballot counting had gone smoothly barring a few glitches, but it criticised state media for lacking any degree of balance during the campaign.
Despite the early lead for Sata, a vocal critic of Chinese mining investment, Banda is expected to close the gap as the countryside constituencies where he counts his major support start to deliver their results.
As such, the race may go right to the wire -- as it did in a 2008 run-off which Sata lost by just 35,000 votes, or two percent of the electorate.
Sata toned down his rhetoric against foreign mining firms in the closing stages of the six-week campaign but a victory for him would still cloud the investment outlook for what has been one of frontier Africa's most attractive prospects.
Political risk analysts, who were divided on the final result, said a Sata defeat might trigger unrest although added that it was likely to be short-lived and have no impact on copper output or the wider economy.
If Sata loses a close election and rejects the outcome there will be some unrest in Lusaka and the Copper Belt but it will be contained, research firm Eurasia said.