Zambia's opposition leader Michael Sata held on to his lead over incumbent Rupiah Banda, the election commission said, as counting moved beyond a halfway point on Thursday in the race to become the next president of Africa's biggest copper producer.

Police said riots had broken out in the two main towns in the Copper Belt as people expressed frustration at a delay in tallying the results of Tuesday's election.

They are on the streets with stones and we can only urge them to stop the riotous behavior, Copper Belt police chief Martin Malama said.

We have not assessed the extent of the damage yet because we are still trying to contain the situation.

According to the latest confirmed tally from 85 of 150 constituencies, opposition leader Michael Sata had 639,787 votes against 542,362 for incumbent President Rupiah Banda of the Movement for Multi-party Democracy (MMD).

The vote count has been slowed by hackers attacking the website of the Election Commission, posting a string of false results showing Sata in the lead and causing delays to the release of the official tally.

It may now take until the weekend for a complete result to be known.

The process appears to be slow because the results need to be verified before release, Commission spokesman Cris Akufuna said. Election officials are running double and triple checks with regional counting centers.

Banda is expected to perform more strongly in the countryside, which is likely to report votes more slowly than Sata's strongholds in the capital, Lusaka, and the northern Copper Belt, the country's economic heartland.

It is therefore too early to say whether Sata is on the verge of an historic transfer of power in the former British colony, removing the MMD from the presidency for the first time since the end of one-party rule in 1991.

Results are still being tallied in some constituency centers, and we expect to make significant progress by the end of the day, Akufuna said.

Sata, known as King Cobra for his vicious tongue, has toned down his rhetoric against foreign mining firms, most notably those from China. But a victory for the 74-year-old would still cloud the investment outlook for what has been one of frontier Africa's most attractive prospects.

He lost to Banda, also 74, by just 35,000 votes, or 2 percent of the electorate, in a 2008 presidential run-off triggered by the death in office of Levy Mwanawasa.

Political risk analysts said a Sata defeat might trigger unrest although added that it would be short-lived and have no impact on copper output or the wider economy.

(Writing by Ed Cropley; Editing by Jon Herskovitz)