Security forces have killed scores of protesters in Syria after mass demonstrations against President Bashar Assad's authoritarian regime were held across the country Friday.

Confirming the number of people killed at the rallies is difficult because the Syrian government, one of the Middle East's most oppressive regimes, has banned foreign journalists and tightly controls access to areas in which there is unrest.

However, activists in the country told The Associated Press that 75 people had died, with many others missing. BBC News said at least 72 people were killed.

It was likely the deadliest day for pro-democracy protesters in Syria since an uprising began five weeks ago.

Large demonstrations were held after Muslim prayers in at least six cities, and were met with live bullets and tear gas from Syrian security forces according to witnesses and human rights groups.

An 11-year-old boy was among those killed, according to one witness who spoke to AP on condition of anonymity due to safety concerns. The boy, whose name was Anwar Moussa, was shot in the head, the witness said.

"Bullets started flying over our heads like heavy rain," said a witness in the southern village of Izraa, where 10 people were reported killed.

Tens of thousands of protesters marched in Douma, a suburb of the capital Damascus, and several other cities. At least five corpses with gunshot wounds were brought to Hamdan hospital near Damascus.

The head of Syria's National Organization for Human Rights, Ammar Qurabi, said nine more people died in Hajar Aswad, near the capital, while three others were killed in the city of Homs.

Friday's widespread unrest suggests that Assad's campaign to quell the month-long protest movement through lethal force and promises of political reform have been unsuccessful.

On Thursday, Assad tried to head off the protests by lifting decades-old emergency laws that gave Syrian authorities sweeping powers to monitor and arrest residents. He has also granted citizenship to thousands of people among the country's Kurdish minority, fired local officials, released prisoners and replaced his government in an attempt to quell the uprising.

But those concessions may have further emboldened the protesters, many of whom claim credit for coaxing Assad to act.

"The state of emergency was brought down, not lifted," Suhair Atassi, a well known Syrian activist, wrote on Twitter. "It is a victory as a result of demonstrations, protests and the blood of martyrs who called for Syria's freedom."

Despite a violent crackdown that has accompanied Assad's political reforms, protesters had pledged to stage the largest demonstrations against his regime yet on Friday.

Assad, who took over from his father as the country's autocratic ruler 11 years ago, faces his largest challenge to date in the popular uprising, which draws its inspiration from similar movements across the Arab world.

But Syria is a unique case in the so-called "Arab Spring" because the military is believed to be loyal to Assad. What happens to Syria also has significant implications for other countries in the region, because of the Assad regime's alliances with Iran and Hezbollah in Lebanon.

Meanwhile the demonstrations have been swelling in numbers, with more and more protesters calling for an end to the regime, rather than political reforms.

More than 250 people are believed to have been killed in the uprising since it began, according to human rights groups.