Syria's main opposition group accused the regime of using chemical weapons on Wednesday to strike rebel areas near Damascus in a "massacre" that left more than 1,300 people dead.
Videos distributed by activists, the authenticity of which could not immediately be verified, showed medics attending to suffocating children and hospitals being overwhelmed.
More footage showed dozens of people laid out on the ground, among them many children, some of them covered in white sheets.
The claim of a chemical attack, which could not be independently verified, was vehemently denied by the Syrian regime which said it was intended to hinder the mission of UN chemical weapons inspectors now in the country.
The Local Coordination Committees (LCC), a network of activists, reported hundreds of casualties in the "brutal use of toxic gas by the criminal regime".
And in videos posted on YouTube, the Syrian Revolution General Commission, another activist group, showed what it called "a terrible massacre committed by regime forces with toxic gas."
The attack "led to suffocation of the children and overcrowding field hospitals with hundreds of casualties amid extreme shortage of medical supplies to rescue the victims, particularly Atropine," the LCC said.
Eastern Ghouta "was also shelled by warplanes following the chemical attack that is still ongoing, which led to hundreds of casualties and victims, among them entire families," it said.
In one video, children are seen being given first aid in a field hospital, notably oxygen to help them breathe. Doctors appear to be trying to resuscitate unconscious children.
"Genocide! Genocide in the town of Moadamiyet al-Sham [southwest of Damascus] using chemical weapons," cries the man behind the camera.
His voice trembling with fear, he adds: "Where are my parents? Where is my father? My mother?"
"People working in the field hospital are overwhelmed and unable to do anything for the wounded. There is a severe lack of medicines. The wounded are being treated using just water and onions," Abu Jihad, an activist in Irbin, told AFP via the Internet.
"This regime considers children in liberated areas to be its enemies," Eastern Ghouta-based Abu Nadim said via the Internet.
The opposition National Coalition's George Sabra, who spoke to reporters in Istanbul, labeled the attack as a "coup de grace that kills all hopes for a political solution in Syria".
The Syrian Observatory for Human Rights, a Britain-based monitoring group, earlier said at least 100 people were killed in air raids and shelling that struck several rebel areas east and southwest of Damascus.
"This figure will surely go up. The raids and bombardment are continuing," said the Observatory.
It did not comment on the allegations that the army had used chemical arms in its assault on the densely populated Ghouta suburbs, where rebels have been holding out against government forces.
State news agency Sana said "reports on the use of chemical weapons in Ghouta are totally false. It's an attempt to prevent the UN commission of inquiry from carrying out its mission."
The agency described Wednesday's violence as "a series of operations" by army units "against terrorist groups" in Jobar, Irbin and Zamalka, "killing a number of them and destroying their hideouts".
In a statement, the army flatly denied as "null, void and totally unfounded" the opposition's allegations, describing them as a "desperate bid to conceal their failures on the battlefield".
Throughout the morning, activists reported chemical attacks in several rebel areas near Damascus.
The Coalition called for an urgent UN Security Council meeting.
"I call on the Security Council to convene urgently," Coalition leader Ahmad Jarba told Al-Arabiya television, condemning what he called a massacre.
British Foreign Secretary William Hague said his country will refer the charges of a chemical weapons strike to the Security Council.
He was "deeply concerned" by the reports and said if they were proved they would mark a "shocking escalation" in the 29-month civil war.
The heavy bombing could be heard in the capital itself, where a grey cloud capped the sky.
The Observatory called for inspectors to hastily visit the stricken sites and ensure access for medical aid.
And the Arab League urged the inspectors to visit the site immediately "to see the reality of the situation and investigate the circumstances of this crime."
UN chief Ban Ki-moon insisted Monday that the inspectors be granted unrestricted access to Syrian sites.
"In order to credibly establish the facts, the mission must have full access to the sites of the alleged incidents," the secretary general said.
Al-Watan newspaper said the government had "pledged to cooperate and facilitate the work" the UN inspectors, who launched their mission Tuesday.
There have frequent claims by anti-regime activists of the alleged use of chemical weapons by the army, particularly in Damascus province and Homs in central Syria.
Claims have also emerged from Idlib in the northwest, while the army and rebels have exchanged blame over the alleged use of chemical arms in a March attack in Khan al-Assal in the northern province of Aleppo.