The Department for Business was blockaded over its continued military contracts with Israel last night, as protestors demanded a total military boycott of the country.
The action came as Britain abstained from a UN vote establishing an investigation into war crimes in Gaza.
Over 25 protestors managed to get inside the building in the evening and blockade the doors.
Demonstrators then put up sheets of papers with the names of the 650 Palestinians who had been killed at that point in the military intervention.
Since then, the number has soared past 690, including one child every hour for the last two days, according to Save the Children.
"Israel has been deliberately targeting civilians and entire families have been killed. What more has to happen before the UK agrees to stop arming Israel?" a London Palestine Action campaigner said.
"Rather than supporting Israel's brutal apartheid system by sanctioning arms exports to Israel, the UK government should be imposing a military embargo and sanctions."
Among other items, the UK sells component parts for sniper rifles and small-arms ammunition to Israel, along with cockpit displays for F-16 combat aircraft and engine assemblies for Apache helicopters.
The Department for Business' own rules mean export licences can only be granted where there is a clear risk the hardware might be used to "provoke or prolong conflict within a country" or "be used aggressively against another country".
But Britain is unlikely to take radical action on its military contracts, because bilateral military cooperation is strongly intertwined, with several years-long contracts keeping the countries bound together.
For instance, the Watchkeeper combat drone is made in the UK by Israeli and French companies, with a £800 million contract with the Ministry of Defence lasting until 2040.
"Hundreds of civilians have been killed using drone technology in recent weeks," a campaigner said yesterday.
"Israeli military companies use Gaza as the testing ground for new weaponry and then make millions exporting the technology."
Britain's reluctance to take action against Israel was evident yesterday when it abstained from a UN vote launching a probe into war crimes during the military action.
Only the US opposed the resolution – as it does any resolution critical of Israel – but Britain joined other EU countries, including France, Germany and Italy, in abstaining.
The resolution passed, with 29 countries voting for it, 17 abstaining and the US opposing.
Foreign secretary Phillip Hammond insisted the resolution was unhelpful.
"Today's UN Human Rights Council resolution will not help achieve a lasting ceasefire," he said.
"It is fundamentally unbalanced and will complicate the process by introducing unnecessary new mechanisms."
However, Hammond told Israeli prime minister Benjamin Netanyahu that Britain was "gravely concerned" by the number of civilian casualties during a joint press conference earlier today.
He will reportedly inform Netanyahu later that the West is losing sympathy with Israel because of its military tactics.
But the majority of Hammond's comments were supportive to Israel, leading Netanyahu to say: "I thank you for keeping your moral focus and your moral clarity. We shall need it in the days ahead."
As the UN vote was taking place, Israel bombed a Gaza hospital which it said was being used as a Hamas base.
Hospital managers said the building, which housed a rehabilitation centre for the elderly and physically disabled, had been evacuated last week after coming under attack several times.
They denied it was harbouring militants, with the executive director of the charity which runs it saying Israel had used "false information" to target the hospital from the start of the intervention.
It was the second hospital to be hit this week, after the al-Aqsa hospital came under Israeli bombardment in an attack which reportedly killed four people and wounded 70 others.
Navi Pillay, UN high commissioner for human rights, said: "These are just a few examples where there seems to be a strong possibility that international humanitarian law has been violated, in a manner that could amount to war crimes."