Uganda's opposition complained of intimidation and interference by the army in their strongholds on Thursday, as voters took part in the country's first multiparty elections for 25 years.

The security forces turned out in large numbers near polling stations, with police expressing concern that there might be an attempt to disrupt the vote.

President Yoweri Museveni, a former guerrilla general who seized power 20 years ago, faces a challenge from his former personal physician, Kizza Besigye, in the first polls since the country's "no-party" system was scrapped last year. Foreign observers noted minor problems with the vote, including the late arrival of ballot slips.

But Sam Akaki, chief spokesperson for the opposition Forum for Democratic Change, said: "Reports are coming in of massive army deployments in every constituency. Tens of thousands of names have disappeared from the register. Ballot papers are arriving late or not at all.

"There are reports of acid being thrown into ballot boxes. At Ssembabule, the army turned up at 10 this morning and carried away the ballot boxes. At Agago in the north they sent only a quarter of the expected ballot papers." The opposition spokesperson said the mere presence of soldiers at polling booths was enough to scare away some opposition supporters.

Officials from the ruling party dismissed the opposition claims as the protests of "bad losers".

As tallies started coming in from 20 000 polling stations, local KFM radio said Museveni was ahead of Besigye by 53,5% to 42.9%, based on 1% of votes cast.

"It's looking like a much closer race than predicted," a senior western diplomat said. "It seems like Museveni is running well behind his results in 2001."

The turnout was said to be high and there were no reports of violence, but thunderstorms forced some outdoor polling stations to close briefly.

In the capital, Kampala, Olive Namtongo (29) explained why she had voted for the opposition. "The country needs change ... he is running out of ideas," she told Reuters. But government supporters fear a return to the bloodstained years of Idi Amin and Milton Obote, when the army was the channel for brutal repression. The president stresses that he maintains a tight grip on the military. A supporter, James Muwanga (32) said: "He has brought peace to this country."

Museveni has also campaigned on his economic record: the economy has grown by 6% a year for the past 10 years. But Uganda remains heavily dependent on agriculture, and a third of the population lives on less than a dollar a day. Western donor countries, which fund nearly half the government budget, are furious that Besigye was dragged through the courts on treason and rape charges when he returned from exile last year.