Karzai told the North Atlantic Treaty Organisation's governing political body on Wednesday: "My request to you today is that you continue to stay with us after the parliamentary elections because if you don't do that you will leave the work half way undone.
"It will take our country many years before we can stand on our own feet in real terms," he added, speaking at the military alliance's headquarters.
More than 10.5 million voters are expected to vote on 18 September to elect the 249-seat lower house known as the Wolesi Jirga.
At the same time, they will chose provincial councils spread over Afghanistan's 34 provinces.
Nato Secretary-General Jaap de Hoop Scheffer highlighted support the alliance has already provided in Afghanistan, and said it aimed to beef up forces under its command in the country.
Nato currently operates the International Security Assistance Force (ISAF) deployed in Afghanistan, which includes about 8000 troops from 47 countries. It is currently active in the north and west of the country.
But de Hoop Scheffer said that the alliance aimed to see the operation extended to the east and south of Afghanistan by mid-2006.
Karzai, on an official three-day visit, is also to meet Belgian Prime Minister Guy Verhofstadt and EU officials.
Karzai was elected president on 9 October 2004 in Afghanistan's first democratic elections.
In Strasbourg on Tuesday, he spoke before the European parliament and called on the European Union to keep up its support for several more years.