Updated

Afghan police and soldiers manned checkpoints at almost every intersection Thursday, searching vehicles and frisking drivers in a massive security operation ahead of elections to choose a new president to guide the country after international combat forces withdraw.

Insurgents fighting the Western-backed government have intensified attacks ahead of Saturday's runoff vote, and the Taliban issued a new statement warning voters to stay away from the polls. The first round in April passed relatively peacefully, but a recent assassination attempt against one of the two presidential hopefuls left in the race has caused fears of more violence to spike.

"The Islamic Emirate deems it necessary to alert the people and warn them for the last time that they should not participate in this American process, deliberately or inadvertently," the Taliban said Wednesday in a statement posted online.

Still, the senior U.N. envoy for Afghanistan expressed confidence Afghan voters would turn out as they did in the first round to decide their future by picking a new leader to oversee the transition after most U.S. and allied forces pull out by the end of this year.

Jan Kubis, the U.N. secretary general's special representative for Afghanistan, also called on the candidates to give electoral authorities time to tally the ballots -- most of which will come from remote regions, often transported by donkeys -- and resolve any complaints amid widespread fear of fraud.

"Give a chance to due process, respect the work of the Commissions, don't jump to conclusions," he said. "Don't make statements or comments in anticipation of the results. it will just mislead the people. control yourself, act as responsible politicians."

He was referring to the likelihood that Abdullah and Ahmadzai's campaigns will start releasing their tallies from monitors at polling sites before formal results are announced. The official timetable is for preliminary results to be announced on July 2 and final results on July 22 in order to allow time for ballots to be secured and fraud complaints investigated.

The stakes are high as the winner will replace President Hamid Karzai, a one-time U.S. ally whose relations with Washington have soured, in the first peaceful democratic transfer of power in the country's history. Karzai has governed the country since the Taliban were ousted following the U.S. invasion in 2001. Karzai but is constitutionally barred from seeking a third term.

The Obama administration also is watching closely. Both candidates have pledged to sign a security pact with the U.S. that would allow thousands of international forces to stay in Afghanistan in a largely training and advisory capacity. Karzai has refused to sign it.

Afghan security forces were widely praised for the April 5 elections, which were held without major violence despite a series of deadly attacks in the weeks beforehand.

Karzai held a video conference with commanders Thursday to urge them to remain impartial and refrain from interfering in the second round balloting.

"All forces must provide the opportunity for the people to cast their votes," he said, according to a statement released by the presidential palace.

Traffic was snarled even more than usual on the streets of Kabul as police set up extra checkpoints and set up extra barriers on many roads to allow only one car through at a time. They also searched many drivers and passengers for possible explosives or other weapons.

The Afghan Interior Ministry also announced that it was banning most trucks and people from other provinces from entering the capital on Election Day.

"Trucks loaded with vegetables that are in danger of being spoiled will be allowed to enter the city after a very careful search process by police," it said.

The Cabinet also has approved a week off for school and university students that began Tuesday because of security issues.

Violence, meanwhile, continued.

A suicide bomber on a motorcycle killed a police officer Thursday in the southern city of Kandahar, according to Dawa Khan Menapal, a spokesman for the Kandahar provincial government.