Armed police in Zambia's capital Lusaka have used teargas to quell protests by opposition supporters against interim results from last week's election.
A number of protesters in Lusaka were arrested
They took to the streets after it was announced that opposition leader Michael Sata had slipped from first to third place with most results in.
The electoral commission said President Levy Mwanawasa has moved into first place with a commanding lead.
President Mwanawasa has appealed for calm in an address on state television.
As his once-strong lead in the presidential vote evaporated, Mr Sata demanded a recount, saying there were serious discrepancies.
With results in from 120 of 150 constituencies, the electoral commission said that President Mwanawasa of the Movement for Multiparty Democracy had just over 42% of the vote.
Another opposition candidate, Hakainda Hichilema of the United Democratic Alliance, had 28%.
And the Patriotic Front's Mr Sata had slipped to 27%.
As the interim results were announced, supporters of Mr Sata began to protest outside Lusaka's main vote counting centre.
Armed police moved in to disperse them, firing teargas and making arrests.
Patriotic Front supporters then set up roadblocks with burning tires and fought with police in several outlying areas.
Protesters clashed with police in several areas of Lusaka
Several shops were looted in these suburbs, Reuters news agency reports.
The electoral commission said it would delay announcing more results until Monday because of the violence.
The head of the electoral commission, Ireen Mambilima, said she had received complaint from both the Patriotic Front and the United Democratic Alliance.
Mr Sata has said as many as 400,000 votes appear not to have been counted in areas where he expected to do well.
The violence and allegations of irregularities contrast with the voting on Thursday, which international observers praised as being generally efficient and transparent.
Officials said the turnout from Zambia's four million registered voters was high.
Mr Mwanawasa's campaign for a second and final five-year term was based on his strong economic record.
But revival in the crucial copper sector is slow, unemployment is high and there is concern over health and education standards.
Mr Sata had vowed to secure a more equitable distribution of wealth if elected.