Syria endured its bloodiest day yet of the Arab Spring as protests against President Bashar al-Assad brought turmoil to dozens of towns and cities across the country and security forces reportedly gunned down dozens of people.

Despite a string of government concessions earlier in the week, including the lifting of the hated 48-year-old emergency law, tens of thousands of demonstrators demanding greater political freedom and an end to Ba'ath party rule took to the streets after Friday prayers.

Security forces around Damascus and other key cities ignored appeals to eschew violence, opening fire with live rounds and using teargas against several pro-democracy protests, activists and witnesses reported.

Although information was difficult to obtain, at least 88 people were reported killed, including two in Douma, one in Homs, and at least six in the southern town of Izraa, and others in Moudamiya, outside Damascus, the activists said.

With more casualties being reported by the hour, there were fears the final toll would be significantly higher. The White House urged Damascus to follow through on promised reforms. Barack Obama called on the Syrian government to stop using violence against demonstrators and accused Assad of seeking help from Iran.

"This outrageous use of violence to quell protests must come to an end now," Obama said.

"Instead of listening to their own people, President Assad is blaming outsiders while seeking Iranian assistance in repressing Syria's citizens through the same brutal tactics that have been used by his Iranian allies."

Foreign secretary William Hague condemned the "unacceptable killing of demonstrators" and called on Syrian security forces "to exercise restraint instead of repression". He said the authorities should respect the "people's right to peaceful protest".

More than 280 people are believed to have been killed since the unrest began six weeks ago. Twenty-one protesters were killed this week in Homs. Activists and observersin Damascus who described today's events as a watershed moment, said their impression was that protests had been bigger than on the past seven Fridays and more bloody.

The protesters' demands varied from place to place. In Kisweh, near Damascus, people called for freedom. In the Mediterranean city of Banias, they chanted: "The people want to topple the regime." Other protesters directed their anger directly at members of the ruling family. "God, freedom and Syria only. God is greatest!" was another rallying cry. In some Damascus neighbourhoods, statues and posters of Assad and his late father, the former president Hafez al-Assad, were torn down, and there was chanting against Maher al-Assad, Bashar's younger brother, who commands the army's elite 4th division. Its soldiers, regarded by many Syrians as a private militia, have been reportedly responsible for shootings in Deraa and elsewhere.

In the Damascus district of Midan, 2,000 people chanted: "Zanga zanga, dar dar, Maher is a bigger moron than Bashar!" Another Assad family member, Rami Makhlouf, a business tycoon who is the president's cousin, was also a target of the protesters' wrath.

Fulfilling an earlier vow to step up the protests on what they called "Great Friday", demonstrators also rallied in the eastern towns of Deir al-Zor and Qamishli.

In the city of Hama, where Hafez al-Assad ruthlessly crushed an armed Islamist uprising nearly 30 years ago, a witness told Reuters that security forces opened fire to prevent protesters from reaching the Ba'ath party headquarters. "We saw two snipers on the building. None of us had weapons. There are casualties, possibly two dead," the witness said.

After prayers finished in Deraa, where the protests first began on 15 March, several thousand protesters gathered, chanting anti-Assad slogans. "The Syrian people will not be subjugated. Go away doctor [Assad]. We will trample on you and your slaughterous regime," they shouted.

Unrest was also reported in Raqqa, close to Damascus, Sayda Zeinab, Harasta and Barzeh in Damascus, Tartous, a coastal town, the western port city of Latakia, and the north-eastern towns of Ras al-Ayn, Amouda and al-Hassakeh.

The scale and nationwide reach of Friday's protests suggested Assad's concessions, far from defusing popular discontent, may have been seen as a sign of weakness by demonstratorsnow doubly determined to achieve their aim. But there is as yet no clear agreement on what their aims are: accelerated democratic reform, greater economic opportunity, an end to corruption among Syria's wealthy elite, or all-out regime change.

In a sign of improving opposition organisation, activists co-ordinating the protests demanded the abolition of the Ba'ath party monopoly on power and the establishment of a democratic system.

They did not call for Assad to stand down.

In their first joint statement, seen by the Guardian, the self-styled "local co-ordination committees", representing provinces across Syria, said that "freedom and dignity slogans cannot be achieved except through peaceful democratic change".

"All prisoners of conscience must be freed. The existing security apparatus has to be dismantled and replaced by one with specific jurisdiction and which operates according to law."

On Thursday, Assad signed a decree lifting the emergency law, imposed by his Ba'ath party when it took power in a coup 48 years ago. He also replaced the cabinet and approved new rights of peaceful protest. But other laws still give security forces sweeping powers.

But the first application to protest under the new law ended in the temporary detention of the applicant. Fadel al-Faisal from Hassakeh was held for several hours after filing a request to hold a demonstration.

Joe Stork of Human Rights Watch said the reforms "will only be meaningful if Syria's security services stop shooting, detaining, and torturing protesters". Syria officials have blamed armed groups, infiltrators and Sunni Muslim militants for provoking violence at demonstrations by firing on civilians and security forces.

Earlier, Reem Haddad, spokeswoman for the ministry of information told al Jazeera: "I think if the people protest peacefully, if they cause no harm, if they don't burn or destroy, I think [security forces] will allow them to do so [protest], and I think after a certain time they will actually disperse them, tell them to go home."

Asked at what point forces would open fire on protesters, she said: "If they are shot at, which has been the case previously."

While calling for an end to the violence and democratic reform, western and other Arab countries have mostly muted their criticism of the killings and repression in Syria for fear of destabilising the country, which plays a strategic role across the Middle East.

Katherine Marsh is a pseudonym for a journalist living in Damascus