"Participation is amazing. How many places in the world do you see 90 percent of people registering to vote?" UN spokesman David Singh told a press conference.
By Thursday, 8,659,772 voters had put their names on electoral rolls, 41 percent of them women, Singh said.
Security was still a problem in parts of the country, Singh conceded, but he said the high registration figures underlined the "determination of the Afghan people" to vote despite the threat of violence from insurgents.
Some estimates put the number of eligible voters at closer to 10.5 million than the 9.8 million figure the United Nations is using to base its figures on.
Singh acknowledged the population estimates "were not set in stone" and could be out by as much as a million people but said the turnout was still encouraging.
More than 80 percent of the population had registered to vote based on an estimate of 10.5 million eligible voters - substantially larger than a turnout of below 50 percent for elections in many Western democracies.
A regional breakdown of the percentage of voters who have turned up to register in different provinces, shows both how inaccurate the estimates of eligible voters are, and highlights regional disparities between the country's north and south.
In the Panjshir valley north-east of Kabul, 118 percent of estimated eligible voters had registered by July 22, according to the UN.
The valley is a stronghold of the Northern Alliance commanders who resisted the Taleban and allied with the US to topple the hard-line regime in 2001.
In the eastern province of Nangarhar, all polling stations are already closed and 133 percent of registered voters have picked up their registration cards, according to UN figures dated July 22.
Voter registration of more than 100 percent was reported in some parts of the country as the original figures were based on a 1972 census and population estimates projected forward.