HARARE (AP): Eager to vote, Zimbabweans began lining up before dawn Saturday for crucial elections where President Robert Mugabe faces the toughest challenge to his 28-year rule and the opposition is urging its supporters to defend their votes against an alleged fraud plot.

With some queues of up to 500, people said they had got in line as early as midnight. But more than a dozen poll stations monitored had not opened by 7:15 a.m. (0515 GMT), 15 minutes after the scheduled opening.

Tensions rose Friday when the military put on a show of force, with soldiers and police in a convoy of armored personnel carriers and water cannon patrolling through downtown Harare, the capital, and the security chiefs warning against violence.

Mugabe told a final rally Friday that Saturday's vote would show Zimbabweans' opposition to former colonizer Britain, whom he accuses of supporting the opposition.

``Zimbabweans are making a statement against the meddling British establishment,'' he told about 6,000 supporters in Epworth, an impoverished town outside Harare.

Mugabe called for discipline at the polls despite ``provocation from outsiders who are already claiming the elections are not free and fair.''

Running against Mugabe are opposition leader Morgan Tsvangirai, 55, who narrowly lost disputed 2002 elections, and former ruling party loyalist and finance minister Simba Makoni, 58.

Tsvangirai on Friday urged supporters at a rally to stay at polling stations until they closed and the counting began, to help prevent rigging.

``They would not rig in front of you,'' he told about 4,000 people at Domboshawa, a small farming community north of Harare. ``We have won this election already. What's left is for us to defend our vote.''

Zimbabwe's security chiefs, though, are firmly behind Mugabe. Defense Forces commander Gen. Constantine Chiwenga has said his soldiers will not serve anyone but Mugabe, and Friday, the chiefs of police, army, air force, prison service and the intelligence agency told reporters the armed forces were ``up to the task in thwarting all threats to national security.''

In downtown Harare, soldiers on quadbikes and police outriders escorted a convoy of armored cars and water cannon.

``Those who have been breathing fire about Kenya-style violence should be warned,'' the security chiefs said in a statement. Deadly protests erupted in Kenya after December presidential elections so rigged no one knows who won. More than 1,000 people were killed there.

On Thursday, Tsvangirai appealed to soldiers and other public servants not to participate in fraud.

Mugabe also has warned he would crush any anti-government demonstrations.

``Just dare try it,'' Mugabe was quoted as saying in the state-controlled Herald newspaper. ``We don't play around while you try to please your British allies. Just try it and you will see.''

Mugabe blames Britain and Western sanctions for the ruin of the southern African country that once exported food, tobacco and minerals. Today, Zimbabweans struggle to survive inflation in excess of 100,000 percent, crippling shortages of food, water, electricity, fuel and medicine. The British charity Save the Children said Friday the number of children who die before their fifth birthday has more than doubled from 59 per 1,000 births in 1989 to 123 per 1,000 in 2004.

Western sanctions introduced after independent monitors said 2002 elections were rigged involve visa bans and frozen bank accounts for Mugabe and 100 of his cronies, but he has convinced many supporters that they are to blame for the country's woes.

Mugabe's critics, though, say it was the government-ordered, often violent eviction of white farmers from land so that it could be handed over to blacks that doomed the agriculture-based economy.

But Fungai Shangwa, a 30-year-old unemployed mother of two, said land reform was one of the reasons she would be voting for Mugabe.

``The opposition will give back land to the whites,'' Shangwa said _ even though she got no land. Most of the seized farms went to Mugabe's friends, relatives and allies.

If he beats Mugabe, Makoni, once a leader in the ruling part, said his priority would be to restore the rule of law to pave the way for economic recovery, and re-engage with the international community.

``The cure is not big piles of money,'' Makoni said in an interview with The Associated Press Thursday. ``We will need money to deal with the crises of food, energy and water but the solution lies in the revival of our own institutions and production in a climate of constitutional order'' to restore local, regional and international investor confidence.

Tsvangirai and other opponents have said Mugabe should be tried, possibly at an international human rights court, for violations including the massacres of an estimated 30,000 mainly civilians during a campaign to subjugate the minority Ndebele tribe in the 1980s.

Makoni said he would not mount a witch hunt against Mugabe, but he also said he would grant Mugabe no special immunity.

Makoni has shaken up Zimbabwe's politics with his appeal to disillusioned citizens, threatening to take votes from both the opposition and the ruling party.

Makoni and Tsvangirai called a joint news conference Thursday, their first together, to appeal to election organizers and regional observers to stop vote rigging.

They complained they had yet to receive full nationwide voters' lists. But Makoni said partial lists showed enough problems to indicate ``a very well thought out and sophisticated plan to steal the election from us.''

Friday night, monitors from the 14-nation Southern African Development Community said they had observed ``a number of matters of concern,'' which they did not identify, and that they would investigate these and raise them with relevant authorities.

Mugabe also is accused of trying to buy votes by handing out tractors, generators and state-subsidized food.