Wearing his trademark green and purple cloak, Karzai said he wanted to run again "to be at the service of the Afghan people," though he acknowledged there have been "some mistakes" during his five-year term as president.
Karzai's popularity has waned somewhat in recent years, as civilian casualties caused by international military forces have increased and charges of government corruption persist.
But so far no candidates who could challenge Karzai's hold on power have registered for the Aug. 20 election.
One possible challenger, Nangarhar Gov. Gul Agha Sherzai, withdrew his name over the weekend following a four-hour meeting with Karzai. Another possible challenger, Dr. Abdullah, the country's former foreign minister, has said he will run but has not yet filed paperwork. Candidates for president have until Friday to register.
In registering for re-election, Karzai made a major change to his ticket, selecting Mohammad Qasim Fahim - a powerful Tajik warlord who has served as defense minister and interim vice president - as one of his two vice presidential running mates.
In selecting Fahim, Karzai kicked First Vice President Ahmad Zia Massood - the brother of resistance hero Ahmad Shah Massood, who was assassinated by al Qaeda two days before the Sept. 11, 2001 attacks - off the ticket. A spokesman for Sherzai said Massood would have run alongside Sherzai before the Nangarhar governor withdrew.
In a reminder of the country's perilous security, a suicide bombing, a roadside bomb and a militant attack killed 24 people Monday in three separate incidents.
In Zabul province, a roadside bomb exploded against a family riding on a tractor, killing 12 people, while militants attacked a convoy and killed six security guards, officials said.
Karzai entered the registration room flanked by the two men running as his vice presidents - Fahim and ethnic Hazara leader Karim Khalili, Karzai's current second vice president.
The president submitted the forms for all three and answered questions from an elections commission official confirming he had gathered the required signatures of supporters. At the end, a smiling Karzai told reporters he was happy his application had been approved and that he had been worried he might fail the registration process.
Fahim served as Karzai's vice president during the country's interim government that was in place following the ouster of the Taliban in the 2001 U.S.-led invasion but before the country's 2004 presidential vote.
During the 2004 election, Karzai dropped Fahim from his ticket in favor of Massood, the exact opposite path that Karzai took this time.
The Afghan constitution says the country's presidential vote was to have been held sometime this spring, but the Afghan election commission pushed back the date to August, saying it needed time to prepare logistics and for the country's security situation to improve.
That decision meant the country could have faced a potential power vacuum, because Karzai's term was to have expired May 21. However, the Supreme Court decided that Karzai should stay on as president during the summer.
Parliamentarians had threatened not to recognize Karzai after May 21, but more recently more than 100 legislators said they would recognize Karzai as president, meaning the country will likely not face major political upheaval in late May.
Karzai registered just ahead of a Wednesday meeting in Washington with President Barack Obama and Pakistan President Asif Ali Zardari, where the three are to discuss the increasingly perilous security situation in Afghanistan and Pakistan.
The U.S. has increased its focus on Afghanistan this year, shifting its resources away from the Iraq conflict. President Obama is sending 21,000 additional forces to bolster the record 38,000 U.S. troops already in Afghanistan in hopes of stemming an increasingly powerful Taliban insurgency.
Colorado Soldier Killed In Afghanistan Returns Home
The remains of Army Corps Sgt. James D. Pirtle, of Colorado Springs, Colo., were returned in a dignified transfer ceremony at Dover Air Force Base, Delaware on Sunday.
Sgt. James Pirtle, who was born and raised in Colorado Springs, turned 21 years old in January. His family told CBS Affiliate KKTV Pirtle was scheduled to return home from Afghanistan in June.
His mother, Patricia Pirtle, says she's still trying to take it all in. "I still can't, it's still like it's not real," she said.
His family says he was funny, fun-loving, well-liked and popular. "He was very dedicated and very patriotic," said Patricia Pirtle. "He was a soldier and he was defending his country," said his aunt, Pam Ashworth.
Sgt. Pirtle is survived by his parents and two older sisters. He also has a niece and two nephews who had already started making plans for his return.
Sgt. Pirtle was stationed out of Fort Hood, Texas. The family plans to have Sgt. Pirtle buried at the Fort Logan Military Cemetery in Denver.