They were pictured waving the Palestinian flag and carrying hammers.
Staff members on site had to be evacuated and operations temporarily suspended as activists descended on the building and caused damage, the group said.
In a tweet, PAS described Thales as having a “killer weapons factory” which has “a vital link in Israel’s military supply chain” adding, “this time, we’re smashing it”.
The protesters had previously made their way onto the roof in June this year, destroying factory equipment.
Police Scotland said officers are still on site as the demonstration continues.
The activists have been protesting over Thales’s arms productions, which include military drones and armoured vehicles, but have focused on the firm’s involvement in the Watchkeeper drone scheme.
The drone, which is reported to have been made in collaboration with Israel’s Elbit Systems, has been used in Iraq, Afghanistan and Palestine, as well as over British waters, the group claims.
A spokesperson for PSA said: “Those in Gaza will often speak of how the drone’s constant presence, including its persistent buzzing noise, induces anxiety and fear, and adds to the constant state of trauma Gaza lives in.
“Stories like these have prompted activists to ensure the root of the misery – Britain’s arms industry, is struck at through direct action.”
Police Scotland said officers were called to a report of a demonstration within the grounds of a business premises on Linthouse Road, Govan, and that the incident is ongoing.
A spokesperson for the force added: “Police Scotland is a rights-based organisation that puts our values of integrity, fairness, respect and a commitment to upholding human rights at the heart of everything we do.
“We have a duty under the European Convention on Human Rights to protect the rights of people who wish to peacefully protest or counter protest balanced against the rights of the wider community.”
Thales UK has been approached for comment.