KABUL — Police stormed a bank in Kabul on today and killed three insurgents who had taken it over, while a wave of attacks killed at least six election workers around the country on the eve of the presidential election, officials said.

The three-man assault came a day after two militant attacks in the capital, including rockets fired at the presidential palace. It also follows a suicide car bomb explosion in front of NATO's Kabul headquarters Saturday that killed seven, a drumbeat of attacks that would appear to signal the intent of Taliban insurgents and their militant allies to disrupt Thursday's vote.

President Hamid Karzai faces some three dozen presidential candidates at the polls, including his former foreign minister and top challenger, Abdullah Abdullah.

Islamist insurgents have threatened violence against those who take part in the election — a crucial step in President Barack Obama's campaign to turn around the war.

In a sign of how difficult election preparations have been, Afghanistan's chief electoral officer said that 20 percent of election materials hadn't yet been delivered to voting sites less than 24 hours before polls open at 7 a.m. Thursday. Daoud Ali Najafi said Afghan army helicopters would be used to deliver the materials to insecure and hard-to-reach regions.

Insurgents unleashed attacks around the country on poll workers, killing at least six.

In a region generally considered safe, four election workers were killed Tuesday when their vehicle struck a roadside bomb about 20 miles outside the capital of northeastern Badakhshan province.

Najafi said they were delivering materials to a polling station.

The latest attacks were an ominous sign that the Taliban and their militant allies are determined to disrupt Thursday's election.

U.S. Secretary of State Hillary Rodham Clinton said Tuesday that the rise in insurgent violence in Afghanistan reflected a deliberate campaign to intimidate voters.