Partial results from Zambia's presidential election show main challenger Michael Sata holding a lead over incumbent Rupiah Banda.

The Electoral Commission of Zambia said Wednesday that with ballot counting still in progress, Mr. Sata of the Patriotic Front party had captured about 42 percent of the vote. Mr. Banda of the Movement for Multiparty Democracy was second with 35 percent.

Hakainde Hichilema of the UPND party was third with 18 percent.

The commission said the count stemmed from results in 133 out of Zambia's 150 constituencies. It has said it aims to have complete results by the end of the day on Thursday.

Pre-election surveys suggested a close race between President Banda and Mr. Sata, who is making his fourth run for the presidency.

Scattered incidents of violence were reported Tuesday in the capital, Lusaka, but European Union election observers say the vote was conducted in a “correct” manner. EU chief election observer Maria Muniz described the election as fair and transparent.

A civil society coalition also said it was satisfied with how the voting was conducted, saying the Electoral Commission “did their best” to make it easy for people to vote.

But coalition chairman Sam Mulafulafu said he is skeptical the electoral commission will finish counting votes by its self-imposed Thursday deadline.

Mulafulafu said the results are being published very slowly, and he cautioned that this could create doubts about the truth of the numbers. He said voters may begin to wonder why the results are being held back or not being made public as quickly as anticipated.

Opposition parties already have expressed concerns, saying the delay 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.

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.