Britain's ambassador to Syria has hit out at the regime's “shocking” violence against its own people.
Simon Collis described seeing security forces beat 40 peaceful protesters in the centre of the capital.
The intervention came as David Cameron pledged to maintain the pressure on Bashar Assad and his government.
In a blog on the Foreign Office website, Mr Collis - who was recalled from Damascus for consultations earlier this week - wrote: "They made no provocative chants and advocated no violence. They simply held up pictures of their friends and family members that had been held in detention for months or years without trial. It was a scene of dignified and peaceful protest.
"After 10 minutes, the regime had had enough. Plain-clothed security forces moved in en masse. We stood and watched as they beat innocent civilians with sticks and batons."
Mr Collis went on: "Without context, it can be hard to make sense of YouTube images shot on a mobile phone.
"It can be hard to understand why a man with a family in a town in Syria would decide to take up arms against his government.
"It can be hard to believe that over 5,000 people have been killed in 10 months, or that torture is a regular occurrence in prisons, children brutalised and tanks and mortars used by the army against its own citizens.
"If I hadn't seen for myself what the Syrian regime has done I would be asking these questions too.
"But I have. And it is too shocking to ignore."
Speaking at an international gathering in Sweden, Mr Cameron said he was determined to see the "toughest possible response" to the brutality.
"It really is appalling, the scenes of destruction in Homs, and it is quite clear that this is a regime hell-bent on killing, murdering and maiming its own citizens," he said.
"Which is why we need to take the toughest possible response we can - a tough response at the United Nations, which we have, and I was very disappointed obviously by the Russian and Chinese veto.
"We also need to work with the opposition to try and help shape their future and assist them in whatever way we can.
"And we also need to put together the strongest possible contact group of like-minded nations.
"We will continue the pressure upon this regime."
The Syrian army has reportedly launched fresh mortar and rocket attacks in Homs, killing scores of people, as the government continues a push aimed at crushing rebel forces.
The city has been a leading focus of unrest in the 11-month uprising against President al-Assad.
UN chief Ban Ki-moon has condemned the "appalling brutality" of the onslaught.