Six people were shot dead and dozens wounded in protests in Afghanistan which flared for a second day on Wednesday in several cities over the burning of copies of the Koran, Islam's holy book, at NATO's main base in the country, officials said.

The American embassy said its staff were in lockdown and travel had been suspended as thousands of people expressed fury over the burning, a public relations disaster for U.S.-led NATO forces fighting Taliban militants ahead of the withdrawal of foreign combat troops by the end of 2014.

The U.S. government and the American commander of NATO-led forces in Afghanistan apologised after Afghan labourers found charred copies of the Koran while collecting rubbish at the sprawling Bagram Airbase, about an hour's drive north of Kabul.

The apologies failed to contain the anger. Thousands of Afghans took to the streets again, chanting anti-American slogans and some raising white Taliban flags in the capital.

Winning the hearts and minds of Afghans is critical to efforts to defeat the Taliban. Similar incidents in the past have caused deep divisions and resentment among Afghans towards the tens of thousands of foreign troops in Afghanistan.

Seven foreign UN workers were killed during protests that raged across Afghanistan for three days in April 2011 after a U.S. pastor burned a Koran in Florida.

A senior Afghan security official, citing reports from police, told Reuters that Western security contractors working at a U.S. military camp in Kabul opened fire on protesters and wounded several.

Witness Rahimullah, 17, said his brother, Ghafar, 23, was shot by one of the contractors in the right leg when he was throwing stones during the demonstration.

He is right now in Daoud Khan Hospital, Rahimullah said of the central Kabul hospital.

The Afghan Interior Ministry said it had ordered an investigation into the shooting.

As a result of shooting at protesters by foreign guards at Camp Phoenix, one person was killed and 10 others were wounded, the ministry said in an emailed statement.

Later, wounded protesters along the busy Jalalabad road on the fringe of Kabul said Afghan police had fired on them.

CULTURAL SENSITIVITIES

Twenty-one people, including 11 policemen, were wounded in the capital, said Mohammad Zahir, head of Kabul police's crimes unit. They included the city police chief, Ayoub Salangi, who was hit in the ankle by a stone.

In Parwan province, home to Bagram, four people were shot dead by Afghan police and 10 were wounded while attacking offices, provincial officials and the interior ministry said.

A protester was shot dead by police in Logar province, east of the capital, the governor's spokesman, Deen Mohammad Darwish, said. Hundreds protested in front of the governor's office. Some threw stones.

Afghan health ministry spokesman Ghulam Sakhi Kargar said one person also died in hospital in Kabul from gunshot wounds received during one of two shooting incidents at protests in at least four areas of the snow-bound capital.

Critics say Western troops often fail to grasp the country's religious and cultural sensitivities. Muslims consider the Koran the literal word of God and treat each book with deep reverence.

Demonstrations by as many as 2,000 people broke out as word of the Bagram find spread.

Police said most injuries were caused by flying stones and sticks hurled by protesters. Demonstrators had charged police lines and nearby military bases at a protest on the edge of Kabul, burning tyres and smashing vehicles and building windows.

Protesters shouted Death to America! and Death to (President Hamid) Karzai as black smoke rose over a large demonstration on the outskirts of the Afghan capital.

When the Americans insult us to this degree, we will join the insurgents, said Ajmal, an 18-year-old protester in Kabul.

Demonstrators set fire to part of a housing compound used by foreign contract workers. A Reuters witness said the fire damaged part of a guesthouse at the Green Village complex, where 1,500 mostly foreign contractors live and work.

Outrage also spilled over in the Afghan parliament, where several members shouted death to America inside the legislative chamber.

The protests spread to several cities.

In Jalalabad, to the east, some protesters burned U.S. flags and shouted Death to America. Others set ablaze fuel tankers near the city's airport.

Demonstrators praised the leader of the Afghan Taliban, the secretive Mullah Mohammad Omar, screaming Long live Mullah Omar!, Reuters witnesses said. Five people were wounded, the governor's spokesman said.

In neighbouring Pakistan's largest city Karachi, around 100 Islamic seminary students protested against the Koran burnings.

Pakistan's government should summon the American ambassador and demand an apology. And if he doesn't apologise, he should be kicked out of the country, said Abdul Basit, a protest leader.

Others took a harder line.

No forgiveness for the desecrators of the Koran, a section of the crowd shouted. Only death.

(Additional reporting by Amie Ferris-Rotman in KABUL and Imtiaz Shah in KARACHI; Writing by Rob Taylor; Editing by Michael Georgy and Ron Popeski)