Defence Minister Florence Parly said cruise missiles had been fired, in coordination with the United States and Britain.
However, opposition Labour leader Jeremy Corbyn said May should have waited it out and "not trailed after Donald Trump".
Other opposition leaders joined in the criticism.
In Northern Ireland, the Deputy leader the Democratic Unionist Party (DUP), Nigel Dodds, said: "The Prime Minister has the full authority, on the basis of all the information at her disposal, to order the type of military action which has been carried out this morning and we reject any suggestion that she was not entitled to do so". He said that "bombs won't save lives or bring about peace".
"This legally questionable action risks escalating further.an already devastating conflict".
This is not about intervening in a civil war.
Stop the War, a pacifist coalition once chaired by Corbyn, has called a demonstration outside the British parliament on Monday to protest the strikes.
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"Initial indications are that the precision of the Storm Shadow weapons and meticulous target planning have resulted in a successful attack", it said. However, that has been less the case in recent years.
British MPs voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
"It was right to take the action that we have done in the timing that we have done", she said.
A YouGov poll for The Times newspaper this week indicated that only a fifth of voters believed that Britain should launch attacks on Syrian military targets and more than two-fifths opposed action.
All crews from the Royal Air Force Tornado jets returned safely, Williamson said. The strikes at 0100 GMT were 15 miles (24 kilometres) west of Homs.
Professor Iain Begg, Research Fellow at the European Institute and Co-Director of the Dahrendorf Forum at the London School of Economics and Political Science (LSE), told Xinhua: "A volley of bombs may help the USA and its allies feel they have reacted in a timely and proportionate manner to the undoubted horror of the use by the Syrian regime of chemical weapons, but the inevitable worry will be that they have not thought through what happens next".
Williamson stressed that the British military did "everything possible" to avoid escalation and prevent losses among the civilian population. Labor leader Jeremy Corbyn, who earlier accused the PM of "waiting for instructions" from the USA president, condemned the strike. "It shows a weak Government putting short term political expediency before democracy and in so doing further diminishing the standing of Britain in the world", said Cable.
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The Green Party, which has one MP, said May had "trampled over parliamentary democracy" and demanded a vote in the House of Commons on the strikes.
DUP leader Arlene Foster said the airstrikes were "limited but proportionate and justified".
Tom Tugendhat, the Conservative chairman of parliament's foreign affairs scrutiny committee and a former army officer, said May had "taken the correct decision".
May told her senior ministers on Thursday that the attack in Douma showed a "deeply concerning" erosion of worldwide legal norms barring the use of chemical weapons.
"The government should do whatever possible to push Russian Federation and the United States to agree to an independent UN-led investigation of last weekend's horrific chemical weapons attack so that those responsible can be held to account", he added.
It has been observed in subsequent military deployments in Libya and Iraq and many British lawmakers and voters are deeply sceptical of deepening involvement in the Syrian conflict.
The air strikes were limited in scope, lasting about an hour. Click any of these buttons below!
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