Britain has sent a team of forensic detectives from the Metropolitan Police to Bahrain to help investigate a huge find of advanced bomb-making materials.
Officials say the explosives, which altogether weighed over 100kg, would have had catastrophic consequences.
Some of it was packaged into bombs and combined with hundreds of iron balls to inflict maximum casualties, they said.
A statement by the opposition said it did not condone violence and that bomb-making was "wholly unacceptable".
Clashes between riot police and masked protesters hurling homemade petrol bombs are a regular occurrence in certain Shia-dominated villages.
However, on Monday a senior police official, who asked not to be named, described the step-up in sophistication as "a game changer" and said the devices "could not have been put together without help from outside the country".
The bomb-making materials were discovered in raids last month in the towns of Salmabad and Hamad following a tip-off.
Officials say it is not clear whether the intended target was a crowded area like a shopping mall or whether the explosives were to be used in an ambush site.
The Bahraini authorities have published the names of three suspects on the run whom the pro-government press says had training from Hezbollah in Lebanon.
Bahraini officials suspect privately that Iran has been training anti-government activists but an independent investigation last year into the violence that has erupted in Bahrain in the wake of the Arab Spring found no evidence that Iran was behind the protests in the troubled Gulf state.
Activists from the Shia majority community are demanding democratic reforms and a bigger role in government, with some calling for the overthrow of the Sunni monarchy that has ruled Bahrain for two centuries.
A statement issued on Monday by the political opposition group al-Wefaq said "making bombs is wholly unacceptable" and said it "condemned any violence in the past, present or future".
But it added: "We find it hard to trust this government as they have fabricated claims in the past".
But Britain, it seems, is taking the find seriously, reinforcing its resident police team at embassies in the Gulf with forensic experts sent out from the UK.
Samples of seized materials are expected to be sent for analysis to Scotland Yard's Bomb Data Centre to trace their source.
A counter-terrorism expert told the BBC: "It is very serious indeed; this is almost at the level and style of the dissident IRA."