Riots have broken out in north-central Zambia as opposition supporters protest the slow vote count from Tuesday's presidential election.

Police say demonstrators stoned cars and buildings in the cities of Kitwe and Ndola on Thursday, and set fire to a market in Kitwe. Regional police chief Martin Malama told reporters that police were trying to contain the situation.

The latest results from Zambia's electoral commission show opposition leader Michael Sata leading incumbent President Rupiah Banda 43 to 36 percent, with results in from slightly more than half of Zambia's voting districts.

But the count has been going slower than expected, and observers predict the commission will not meet a self-imposed Thursday deadline for the final results.

Tensions were heightened on Wednesday when Zambia's High Court banned three independent media organizations from publishing speculative reports on the elections' outcome. The injunction came after The Post newspaper ran a headline that read “Sata Heads for Victory.”

Later in the day, the president's office released a statement in which it dismissed “rumors” the final results have been compiled and that President Banda has been informed of the development.

On Wednesday, unidentified hackers attacked the website of Zambia's Electoral Commission, posting results showing Mr. Sata holding a strong lead with nearly all of the votes counted. The results were later taken down.

Analysts say Mr. Sata's early lead has been boosted by a strong performance in Zambia's urban areas, where votes are likely to be counted more quickly. Mr. Banda is expected to perform stronger in the countryside.

Opposition parties have complained that the delay in releasing the results may be a ploy to allow the commission to skew results in favor of President Banda.

Ahead of the poll, Mr. Sata accused the electoral commission of planning to rig the outcome using pre-marked ballots. The electoral commission has denied that allegation.

European Union election observers said Thursday that the elections were “generally well administered,” but unequal access to resources meant there was not a “level playing field” for campaigning.

Specifically, observers criticized state-owned media for failing to meet “even their minimal obligations as public service media,” saying state news programming lacked balance in its coverage of the campaign.

President Banda campaigned on a record of several years of strong economic growth in copper-rich Zambia, which has benefited from a boom in global commodity prices. Mr. Sata's Patriotic Front accused Mr. Banda of tolerating corruption and not doing enough to ensure that more Zambians share in the wealth of the country's copper reserves.

Mr. Banda defeated Mr. Sata by two percentage points in a 2008 special election to complete the term of the late President Levy Mwanawasa, who died of a stroke.

Mr. Banda served as Mr. Mwanawasa's deputy. Mr. Sata disputed those election results, and his supporters rioted for days afterward.

More than 5 million Zambians were registered to vote in Tuesday's presidential, parliamentary and local elections.