Front-runner Abdullah Abdullah escaped an assassination attempt on Friday when two bombs blew up outside a hotel where he had just staged a rally. One of the cars in his convoy was destroyed.
The death toll from the attack in the Afghan capital had been put at six but doubled to 12 by Sunday, with at least 40 people wounded, Reuters reported.
"Security measures have been tightened from last night," Interior Ministry spokesman Sediq Sediqqi said.
The new measures included deploying more security forces to rallies and assigning larger teams of bodyguards to Abdullah, a former foreign minister, and his opponent in Saturday's run-off, former World Bank economist Ashraf Ghani.
The heightened security measures would protect both candidates and their audiences, and would allow the run-off campaign to continue as planned, Sediqqi said.
"We strongly believe nothing will discourage people from voting on Saturday," he said.
No candidate won more than 50 percent of the vote in the first round last month, forcing the election into a run-off, although Abdullah led Ghani by almost 14 percentage points.