Tehran, Iran ? A captive Royal Marine was shown in new TV footage Friday apologizing for being in Iranian waters, and Tehran made public a third letter supposedly written by the only female prisoner among 15 Britons seized by Iran’s Revolutionary Guards.

Britain sharply denounced Iran for the treatment of the captives – a clear sign both sides were hardening their stance as the crisis entered its second week.

Iran appeared intent on sending a message of strength as it faces mounting U.N. sanctions for its uranium enrichment program, which the U.S. and other nations suspect the Islamic Republic is using to develop nuclear weapons.

Underlining Iran’s hard-line sentiment, some 60,000 soccer fans chanted “Death to Britain” at a match in Tehran, while 700 people rallying near Tehran University yelled “We condemn the British invasion!” A Muslim cleric told worshippers during Friday prayers that “Britain is an aggressor and Iran has confronted it.”

In the latest video broadcast by Iranian state television, Royal Marine rifleman Nathan Thomas Summers was pictured while sitting with another male captive, both in fatigues, and female British sailor Faye Turney in a blue jumpsuit and a black head scarf.

The three were among 15 British sailors and marines detained by Iranian naval units March 23 while patrolling for smugglers near the mouth of the Shatt al-Arab, a waterway that has long been a disputed dividing line between Iraq and Iran.

“We trespassed without permission,” Summers said in the video shown on Iran’s Arabic-language channel Al-Alam. “I deeply apologize for entering your waters.”

The video, the second broadcast of the detainees in three days, drew indignation from British Prime Minister Tony Blair, who accused Tehran of manipulating the prisoners.

“All it does is enhance people’s sense of disgust. Captured personnel being paraded and manipulated in this way doesn’t fool anyone,” Blair said. “What the Iranians have to realize is that if they continue in this way, they will face increasing isolation.”

It was not known whether the marine spoke under pressure from his captors, but Summers said in the broadcast that “our treatment has been very friendly.”

Iran also released a third letter supposedly written by the sole female detainee in which she says she has been “sacrificed” by Britain. She said she was sorry for straying into Iranian waters and asked if it wasn’t time for Britain to withdraw its troops from Iraq.

The European Union warned Iran of undefined “appropriate measures” if the British naval team remained in captivity.