A policeman stands at the site after five car bombs detonated simultaneously in Afghanistan's largest southern city of Kandahar. (Allauddin Khilji/Associated Press)

A large blast made by five car bombs strung together rocked Kandahar city Tuesday night, killing at least 41 people, wounding 66 and damaging dozens of buildings, officials said.

Gunfire followed the explosion, which sent flames shooting into the Afghan sky. It targeted a Japanese construction company that mostly hires Pakistani engineers, said Haji Agha Lalai, a provincial council member.

He said the vehicles were filled with explosives and detonated together.

One Associated Press reporter estimated that the blast destroyed about 40 shops.

"Once again, they've killed children, women, innocent Afghans," said deputy provincial police chief Mohammad Sher Shah. "They are not human. They are animals. You can see for yourself the destruction of this enemy."

Speaking at CFB Valcartier in Quebec, Defence Minister Peter MacKay said the blast showed the challenges that continue in Afghanistan.

MacKay said Canada will continue to support the training of the Afghan national security forces and continue to build infrastructure in the country.

"But it's anything but a simple solution, and we know that it is going to take a sustained period of continued focus and energy and investment, " he told reporters.

Earlier Tuesday, four U.S. soldiers were killed in another bomb blast in southern Afghanistan, bringing the death toll of foreign forces in 2009 to 295 — the most in a single year since the start of the war.

Tuesday night's explosion came just hours after Afghanistan's Electoral Commission released partial results from last Thursday's election.

They showed incumbent President Hamid Karzai had a slight lead with 41 per cent of the vote, while Abdullah Abdullah, the former foreign minister and top challenger, had 39 per cent of the vote with 10 per cent of the polls reporting.

Taliban militants have launched several attacks in Kandahar, which is their base in southern Afghanistan, over the past few years.

A large NATO base sits on Kandahar's outskirts, but militants control districts immediately to the west of the city.

With files from The Associated Press