"It was a massive blast that could be heard by people across the city. It has caused widespread devastation in the centre of the city. It is a largely residential area that has been devastated by this bomb blast."
The bomb went off shortly after dusk fell and people were breaking their Ramadan fast.
Ahmed Wali Karzai, a representative of Kandahar provincial council, told Al Jazeera: "There are still some people trapped. We have people trying to get them out of the rubble."
Agha Lalai, another council member, said the blast "felt like an earthquake. The power went off and there was a huge explosion".
The bomb went off near a guest house frequented by foreigners, the Kandahar provincial intelligence headquarters and just 800m from the local UN offices.
"The staff is good, everybody is safe," Samad Khaydarov, head of the UN assistance mission in Afghanistan.
|At least 90 people were injured
in the attack [AFP]
"Our office, our guesthouses, are safe. ... Unfortunately, security is not so good in Kandahar."
Fazel Ahmad Shairzad, the deputy Kandahar police chief, said police did not yet know what the target was and were investigating.
There was no immediate claim of responsibility but a government spokesman blamed the attack on Taliban-linked groups.
Kandahar is the biggest city in southern Afghanistan, an area where Taliban fighters battling the Western-backed government continue to have strongholds.
The city was the powerbase of the former Taliban government, which was ousted from power in Afghanistan by US-led troops in 2001 and replaced with Karzai's Western-backed administration.
The Taliban have struck repeatedly in recent weeks in a bloody countdown to nationwide elections last week, only the second time that war-weary Afghans have voted for a president in their history.
Earlier on Tuesday, four US soldiers operating with Nato were killed in a bomb blast in the south of the country.
The US military said the soldiers had been killed by an improvised explosive device.
The deaths make August the second most deadly month for US troops in Afghanistan since the 2001 invasion so far.
Also on Tuesday, a British soldier who was wounded while on patrol on August 15 in Helmand province, died in hospital, the British defence ministry said.