Thousands of Palestinians in Syria are fleeing Damascus after an attack on the country's largest refugee camp, according to survivors who have reached Lebanon.

Some of those who have made it to the relative safety of Beirut claim the attack marks a "historical moment" in the Syrian war that has shattered the regime's claim to be a patron of resistance against Israel.

The fallout from the attack on the Yarmouk refugee camp in south-west Damascus on Sunday night is now reaching beyond Syria's borders, with Lebanon and Jordan braced for a fresh refugee crisis.

About 1,000 Palestinians had reached Lebanon less than 48 hours after a Syrian jet bombed a mosque and a school inside Yarmouk camp, the first time the large, sprawling section of the capital had been targeted from the air and only the second time it had been struck since the civil war began. The air strike is believed to have killed about 25 people and wounded several dozen more.

The new arrivals say they fear that authority in the Syrian capital is starting to crumble. They are now openly hostile towards a regime that had long portrayed itself as the protector of the 500,000 Palestinians living in Syria, most of whom had called Yarmouk home until now.

"No Palestinian will trust them anymore after what they did on Sunday," said Abu Khalil, a father of three who has taken refuge in the infamous Beirut refugee camp Sabra-Shatila. "All of us accept that blood has been drawn between us and the regime. There is a debt to settle. It will never be like it was."

Abu Khalil and his extended family of 15, now refugees for a second time in a lifetime, say the attack has repulsed Palestinians who had enjoyed the patronage of the Assad regime for more than 40 years but had increasingly been expected to openly align with them.

Abu Khalil offered an account of what took place on Sunday in the hours before the attack and in the frenetic aftermath, which has led to unprecedented criticism of the regime from most Palestinian factions.

"Since the summer, the two intelligence bases in the camp, air force intelligence and political security, were opened as recruitment centres for anyone who wanted to join Ahmed Jibril," he said. "Anyone who did was given a gun."

Ahmed Jibril runs the Popular Front for the Liberation of Palestine–General Command, a faction that has remained loyal to the Assad regime and is hostile to the main Palestinian organisational body, the PLO.

"There had been no fighting inside the camp at all until Sunday," he said. "There were clashes on the outskirts, but the Free Syria Army had not entered the camp at all. They only came in after the air strike."

About 3,000 members of the Free Syria Army and the al-Qaida-aligned Jabhat al-Nusra are now inside the camp, Abu Khalil said. He claimed only 500 residents remain, with most having sought refuge in homes, schools and mosques elsewhere in Damascus.

Jibril had about 1,000 armed men but only 150 of them were fighting with him on Sunday," he said. "They fled after a few hours.

"Some of the rebels who came in after the attack spoke with strange dialects. Others had beards, like jihadists. They were all telling us not to worry. It was the first time we had seen any opposition member in Yarmouk."

Abu Khalil's mother, who called herself Um Hassan, said warnings broadcast from mosques in Yarmouk early on Sunday had given residents two hours to leave.

Many had done just that, she said. However, others had sought refuge in a mosque and remained behind. Syrians who had fled from battlezones elsewhere in Syria were staying in a nearby school. They also chose to stay. Both groups were hit by bombs dropped from jets.

"We left at 7am on Monday and got to Sabra-Shatila at 3.30am [on Tuesday]," said Abu Khalil. "It was the biggest humiliation I have ever felt. We left with only the clothes on our backs.

"Three weeks ago we watched the ugly scenes as the Israelis bombed Gaza. We know what to expect with them. But I can't describe the feeling of Muslims attacking Muslims. It was a historical moment."

Palestinian leaders in Lebanon say they are bracing for the arrival of 50,000 refugees from Yarmouk, an influx that would seriously strain resources inside the country's 12 established camps. Such numbers could also potentially upset the delicate sectarian balance in the still-brittle country, where sect numbers are bitterly contested and often used as political tools.

Unlike in Lebanon, Syria's Palestinians had largely enjoyed equal rights as citizens, with access to homes, healthcare and other trappings of state.

Their treatment has often been showcased by regime officials as a sign of Syria's support for a people who have remained at odds with their sworn enemy, Israel. The regime's far-reaching support for Hezbollah has been the second dimension of its resistance credentials.

The Yarmouk attack is also being seen as a turning point by senior Palestinian officials in Lebanon. Qassem Hassan, the general secretary of the PLO in Sabra-Shatila, said: "We sense a very bad smell to this. Why this is happening, we can't understand. The PLO had taken a position not to support the regime or the other side.

"We did not interfere in the affairs of Syria and they shouldn't have interfered in ours. A volcano has erupted here. Is this part of a plan to reorganise the Middle East? We don't know. But it is a very big event."