In an interview on Iranian television, Mousavi said it was "a form of piracy" that proved Britain was following "the hostile policies of the US".
US National Security Advisor John Bolton said the British move was "excellent news". "America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade," Bolton said on Twitter.
The drama began when a specialist team from the Royal Marines' Maritime Operations Unit swooped on the super-tanker as it paused to take on supplies off Gibraltar.
The 30 commandos inserted on a Pacific 24 high-speed boat and by fast-roping from a Wildcat helicopter, allowing 16 Royal Gibraltar Police officers to search the vessel in a safe manner.
The operation was over in minutes with no shots fired.
Fabian Picardo, Gibraltar's Chief Minister, said he authorised the assault and a police request for Royal Marine assistance because he believed the ship was bound for the Baniyas refinery in Syria.
"That refinery is the property of an entity that is subject to European Union sanctions against Syria," he said.
The Foreign Minister of Spain said Britain had acted as the request of the US. Josep Borrell said Madrid was assessing the implications of the incident because it took place in waters it considers its own. Spain disputes British ownership of Gibraltar.
Britain's Foreign Office said Rob Macaire, the UK ambassador in Tehran, told the Foreign Ministry when he was summoned that the move was about enforcing sanctions on Syria, not on Iran, and that Britain views Iranian oil exports in general as legitimate.
Lloyd's List, the shipping publication, said the 300,000-tonne, Panamanian-flagged tanker loaded oil off Iran in April before sailing around the Cape of Good Hope, a lengthy route apparently taken because the ship was too heavy to use the Suez canal.
The vessel was one of four tankers found to be involved in shipping Iranian fuel oil to Singapore and China in violation of US sanctions by a Reuters investigation earlier this year.
Its seizure will further inflame the difficult relationship between Britain and Tehran, which regularly clash over Iran's foreign policy and the fate of jailed British dual nationals including Nazanin Zaghari-Ratcliffe.
It may also complicate British, French, and German efforts to save the 2015 nuclear deal and to defuse a confrontation between the US and Iran in the Gulf.
Hassan Rouhani, Iran's President, said on Sunday that Tehran will increase its enrichment of uranium to "any amount that we want" after it breached the 300kg limit laid out in the nuclear agreement in protest at US attempts to strangle its oil exports.
Britain officially defends Iran's right to export oil under the 2015 nuclear deal, which offered the Islamic Republic economic incentives in exchange for curtailing its nuclear programme.
But the US, which quit the deal last year, has attempted to blockade Iranian oil exports under a "maximum pressure" policy designed to force Tehran to accept more restrictive limits on its nuclear programme, curtail its ballistic missile programme and abandon covert military operations in the Middle East.
The policy has reduced Iran's exports to well below 500,000 barrels of oil a day, a fraction of the 1.8 million economists believe the country needs to operate.
Iran has said it will continue to breach some of its nuclear commitments unless the remaining signatories of the deal — Britain, France, Germany, the EU, Russia, and China — find a way for it to sell oil and access the revenues. Telegraph Group Ltd
