Eight people have been killed in Afghanistan as protests continue against the burning of the Quran by a controversial US pastor.
Thousands of people took to the streets in Kandahar city on Saturday, a day after a deadly attack on UN staff.
Protesters attacked the police and set shops ablaze. About 70 people were injured.
Abdul Qayoum Pukhla, a senior doctor at Kandahar's Mirwais hospital, said victims suffering from bullet injuries and wounds caused by rocks had been admitted to the hospital.
The spokesman for the governor of Kandahar province said the protest was organised by the Taliban who used the Quran burning in Florida as an excuse to incite violence.
"The demonstration in Kandahar was planned by insurgents to take advantage of the situation and to create insecurity," Zalmay Ayoubi said.
UN office attacked
A day earlier, after Friday prayers ended, protesters overwhelmed security guards at the UN office in the usually peaceful northern city of Mazar-i-Sharif. They burned parts of the compound and climbed blast walls to topple a guard tower.
Afghan officials said at least 11 people were killed, including seven UN staff. The throat of one of the dead foreigners was slit, the UN said.
Demonstrators had gathered to protest over reports that an evangelical pastor last month burned a copy of the Muslim holy book in the US.
The pastor's "Judge the Quran day" drew widespread international condemnation
The Taliban said they had no role in Friday's assault on the UN office, after both the provincial governor and a senior UN official suggested provocateurs among the crowd had sparked or led the vicious attack.
"The Taliban had nothing to do with this, it was a pure act of responsible Muslims," spokesman Zabihullah Mujahid told the Reuters news agency.
"The foreigners brought the wrath of the Afghans on themselves by burning the Quran."
President Karzai spoke with UN Secretary-General Ban Ki-moon on Saturday morning to convey his grief over the attack on the UN.
Karzai described the attack as "ruthless" and affirmed that the government of Afghanistan is committed to launching an all-out probe into the incident and bringing to justice those responsible.
Stressing the importance of promoting a peaceful co-existence and harmony among the religions, Karzai asked Ban to play his role in raising public awareness on the significance of resorting to non-violence and non-desecration of faith and dialogue among religions.
Also on Saturday, fighters clad in burkas attacked a coalition base in Kabul with guns and rocket-propelled grenades, but were killed either when they detonated their explosives or by Afghan or coalition fire outside the entrance, NATO and police said.
Al Jazeera's Hashem Ahelbarra, reporting from Kabul, said the Taliban had claimed responsibility for that attack.
"They're saying the Americans, the Europeans and the United Nations have to take a very tough stance when it comes to the desecration of Islamic symbols and they haven't seen that, they haven't seen any statement issued by the UN or the American forces in Afghanistan," he said.
"This is why they're trying to converge in the main cities, in particular in areas where there are military bases and UN offices, to send a very strong signal that they are really angry about what's going on."
Terry Jones, an American pastor, created a storm of controversy after he announced that he would burn copies of the Quran on the anniversary of the September 11 attacks last year. Under pressure from political leaders, Jones "suspended" the event.
However, on March 20, Jones oversaw the burning of a copy of the Muslim holy book by another pastor, Wayne Sapp.
Many Afghans only found out about it when Karzai condemned the desecration four days later.
Sapp called the deaths in Mazar-i-Sharif "tragic," but said he did not regret the actions of his church.
"I in no way feel like our church is responsible for what happened," he said.
Protests also broke out on Friday in Kabul and Herat in western Afghanistan.