Earlier, the men had been shown sitting silently on chairs and a sofa. Three were in British military uniform; five others wore military trousers and civilian T-shirts.
"They will be prosecuted for illegally entering Iranian territorial waters," Al-Alam television said.
"The vessels were 1,000 meters inside Iranian territorial waters. The crew have also confessed to having entered Iranian waters," the broadcast said. The distance is about a half-mile.
"Even if they have strayed into Iranian water technically by a few yards, in one sense, so what?" said British Conservative Party foreign policy spokesman Gary Streeter.
The British government said the men were on a "routine mission" in the Shatt-al-Arab waterway that separates Iran and Iraq along their southern border. The Foreign Office summoned Iranian Ambassador Morteza Sarmadi to demand an explanation for the naval officers' arrest.
"The ambassador was asked to explain why the eight are being held, for their release as soon as possible and for full consular access to them meanwhile," a Foreign Office spokesman said told the BBC.
British Foreign Secretary Jack Straw has spoken to Iranian Foreign Minister Kamal Kharrazi about the impounding of three British military vessels and the detention of their eight crewmen in the Shatt-al-Arab waterway, the Foreign Office said Tuesday.
"The Foreign Secretary spoke this morning to his opposite number Kharrazi and Kharrazi said he would look into it personally," the Foreign Office said.
The BBC reports the two men have a good personal relationship after numerous meetings in the past.
According to state-run television in Iran, the eight crewman are to be prosecuted. But the British government has not been told so formally.
Streeter wants the government to take a hard line.
"There's no justification for holding these men. They've got to be released," he said.
Diplomatic relations between Britain and Iran have been unpredictable for years, and have been strained recently over Iran's nuclear program, reports CBS News Correspondent Steve Holt.
Britain has pursued a policy of constructive engagement with the clerical regime and last year, with France and Germany, persuaded Tehran to cooperate with international nuclear inspectors.
But relations are again strained, after London helped draft a resolution at last week's meeting of the International Atomic Energy Agency, for failing to meet its commitments.
British diplomats sought to play down a possible link between the resolution and the arrest of the eight crewmen, however, and suggested the arrests had been made by an opportunistic local military commander.
"I hope this doesn't get caught up with the bigger negotiations going on at the moment about the Iranian nuclear capability," said Streeter, the shadow foreign secretary. "The pressure that's being applied on them by the international community has got nothing to do with it whatsoever."
With the June 30 handover of sovereignty to an Iraqi government approaching, some analysts are saying that this is simply Iran asserting its control over the Shatt-al-Arab waterway, which has been a cause of many conflicts between Iran and Iraq, reporter Ramita Navai in Tehran told CBS Radio News. "It was actually a catalyst of the eight-year Iran-Iraq war."
The British Defense Ministry said the personnel from the Royal Navy training team based in southern Iraq was detained while delivering a boat from Umm Qasr to Basra, Iraq, to the new Iraqi Riverine Patrol Service.
The ministry said their boats were unarmed but the crew were carrying their personal weapons.
The Royal Navy has been training Iraqi personnel in coastal defense for several weeks on the Shatt al-Arab waterway, and it is possible that the vessels out of radio contact were taking part in such an exercise.