By Stella Mapenzauswa and Chris Mfula

LUSAKA (Reuters) - Zambia's President Edgar Lungu took a narrow early lead over his main challenger in a tight race for the presidency on Friday after the electoral commission delayed results twice and the opposition accused it of fraud.

The Electoral Commission of Zambia (ECZ) said audits were taking longer than expected partly due to a large voter turnout. It had promised the first results by 1200 GMT and then 1600 GMT.

The opposition United Party for National Development (UPND) said that the ECZ was trying to manipulate the results in favor of Lungu's Patriotic Front (PF) by giving someone unauthorized access to ECZ computers.

UPND leader Hakainde Hichilema said an ECZ official had given his identity card to a man so that he had access to ECZ's IT room and could tamper with results. The ECZ said the intruder had been detained and police were investigating.

Data from three constituencies showed Lungu won 17,620 votes in Thursday's election, against 16,358 for Hichilema.

Campaigning had centered on the economy, after months of rising unemployment, mine closures, power shortages and soaring food prices in Africa's No. 2 copper producer, and supporters of the two main parties clashed in the run up to Thursday's vote.

Hichilema says the president has mismanaged the economy but Lungu blames weak growth in the major copper producer on plunging commodity prices.

Lungu only narrowly won a vote 20 months ago after the death of president Michael Sata. If he fails to win an outright majority this time, he will be forced into a second-round rerun.

The ECZ said it had met with political party leaders to address concerns including over the late results, and denied colluding with the ruling party.

"It hurts the commission when allegations which are unsubstantiated are made. Don't set the house on fire," chairman Esau Chulu said.

Final results would now not be ready for late Saturday or early Sunday as had been expected, he said.

The possibility of a rerun, entailing a further four or five weeks of campaigning, would raise fears of more violence, NKC Africa analyst Gary van Staden said.

"But ... given that Zambia does have strong democratic institutions, that it has a strong and robust civil society and a people with a long history of peaceful engagement, there is reason to remain optimistic," he said.

Zambians had also voted on Thursday for members of parliament, mayors, local councillors, and in a referendum on proposed changes to the constitution.

(Editing by Louise Ireland)