Hundreds of troops, police and security guards, including special American bodyguards, were on alert as Karzai spoke to about 10,000 people in the town of Ghazni, southwest of Kabul, on Tuesday.
Speaking alternately in the country's two main languages, Dari and Pushto, Karzai said: "This vote is not just to choose a president, but for peace and stability in Afghanistan.
"Instead of fighting, we are campaigning for our elections. We should be proud that we have freedom at last."
Afghanistan, battered by 25 years of war since the Soviet invasion of 1979, holds a direct presidential election for the first time on Saturday.
Qanuni was former education
minister in Karzai's government
Eighteen candidates, including Karzai, are in the fray. His main rival, Yusuf Qanuni, a former education minister addressed about 4000 cheering supporters in Kabul's main stadium and said he was the man to back.
"Dear brothers and sisters, you are the ones who will elect the president of Afghanistan," he said. "I want your support, I want your vote."
Afghanistan has not held any form of election since the late 1960s. More than 10.5 million people have signed up to vote in the country itself, from a population of about 28 million.
Almost 750,000 Afghan refugees in neighbouring Pakistan, who fled the violence in their homeland, have also registered. An estimated 400,000 to 600,000 are eligible to vote in Iran as registered Afghan refugees.
Violence is still the overriding concern of the election, however. The Taliban and al-Qaida have vowed to disrupt the poll.
Karzai escaped a rocket attack on his helicopter when he arrived in the town of Gardez to campaign three weeks ago, which was blamed on the Taliban.
"This vote is not just to choose a president, but for peace and stability in Afghanistan"
Officials hope the Taliban will be thwarted by a security effort involving a national army of more than 17,000, about 25,000 police, 18,000 US-led coalition troops and a NATO-led International Security Assistance Force of more than 8000.
Karzai is the favourite, but it is unclear if he will get the 51% majority needed for an outright win. If no one gets a majority, the top two candidates will fight a runoff in November.
Other major candidates include ethnic Uzbek leaders Abd al-Rashid Dostum and Abd al-Satar Serat and Hajji Muhammad Muhaqiq, a Hazara.