KABUL, Afghanistan — Anti-American protests spread across Afghanistan on Wednesday, as demonstrators seethed over the burning of Qurans at a NATO air base, and Afghan politicians demanded harsh punishment for the offenders.

At least seven Afghans were killed and dozens were injured, according to the Interior Ministry, when protesters gathered in several cities, throwing stones, burning tires and lighting effigies of President Obama. One crowd attempted to storm a fortified compound in Kabul where hundreds of U.S. contractors live.

Security forces tried to quell the scattered protests — in some cases by firing on demonstrators — but the unrest showed no sign of dissipating.

In an Afghan parliamentary session, lawmakers backed demonstrators' demands that the offenders be tried in an Islamic court, applying pressure on President Hamid Karzai to act swiftly and assertively in meting out punishment.

Some members of Parliament said the Quran burning was intended as an insult to Afghans. Others urged security officials and soldiers to wage holy war against Americans, a sentiment commonly expressed at the demonstrations.

Meanwhile, Afghan and NATO officials began a joint investigation into the incident amid concerns that the outrage could threaten stability during a critical time in the war.

NATO and Afghan officials visited the Parwan Detention Facility, adjacent to Bagram Air Field, where the incident occurred, to "examine the circumstances surrounding the disposal of religious materials there," according to a news release from NATO-led forces.

Western officials said publicly Tuesday that the books were taken to the incinerator by accident. "I assure you ... I promise you ... this was NOT intentional in any way," said Gen. John Allen, the top U.S. commander in Afghanistan.

But a senior U.S. military official, who asked to remain anonymous, said Tuesday that the Qurans were removed from the prison library because they had radical or anti-Western messages scrawled in them. After issuing an apology Tuesday, Allen promised that all NATO forces in Afghanistan would complete training in the proper handling of religious materials by March 3.

Karzai seemed keen on using the incident to advance his case that Afghans should take responsibility for the Parwan facility, the largest U.S.-run military prison in the country.

"The sooner you do the transfer of the prison, the less you will have problems," Karzai said at a meeting Wednesday with U.S. Deputy Defense Secretary Ashton Carter, according to a statement from Karzai's office.

Last month, Karzai demanded that responsibility for the detention center be handed over to Afghanistan by the end of January. He has since extended that deadline until March 9.

In addition to demonstrations in Kabul, protests were held in Jalalabad, east of Kabul, and in Parwan province, where the Bagram base is located. Police in Logar province, south of Kabul, fired on demonstrators, killing one person, officials said.