Story Highlights• Hundreds of protesters gather outside the UK embassy in Tehran
• "The Iranians must give back the hostages," U.S. president says
• Bush calls detention of Britons "inexcusable behavior"
• U.S. president says sailors were taken from Iraqi waters
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TEHRAN, Iran (CNN) -- Hundreds of Iranian students crowded outside the British Embassy in Tehran on Sunday, setting off firecrackers and hurling projectiles toward the compound, an embassy spokesman said.
No one was injured and there was no damage in the protest, which continued into the late afternoon, the spokesman said.
The students are protesting the alleged trespass of 15 British marines and sailors into Iranian waters on March 23.
Britain and Iraq say the Britons were well inside Iraqi waters, and London is demanding the release of the 15 detainees.
Iran has not allowed British ambassadors access to the Britons, who are being held at an undisclosed location in Iran.
Video from earlier in the day showed Iranians of all ages crowded around the embassy while Iranian forces maintained a cordon around the peaceful crowd, which chanted and waved flags.
U.S. President George W. Bush called Iran's detention of the sailors "inexcusable behavior" and called for their release, referring to them as "hostages."
"The Iranians took these people out of Iraqi water," said Bush, speaking Saturday at Camp David with Brazilian President Luiz Inacio Lula da Silva. "And it's inexcusable behavior."
The U.S. government had been notably quiet on the subject from the beginning, but Bush voiced strong opinions Saturday.
"The Iranians must give back the hostages," he said. "They were innocent. They were doing nothing wrong. And they were summarily plucked out of water." (Watch President Bush's address )
Also Saturday, an Iranian official said his country had started a legal process to determine the guilt or innocence of the detainees.
If they are not guilty, they will be freed, said Ambassador Gholam-Reza Ansari, who is in Russia.
"But the legal process is going on and has to be completed, and if they are found guilty, they will face the punishment," he said on Russian TV. (Watch Iranian ambassador call British sailors 'invaders' )
Ansari -- speaking to the TV news channel Vesti-24 -- also hinted that there could be a diplomatic settlement, according to the Islamic Republic News Agency in Iran.
"If the UK government admits its mistake and apologizes to Iran for its naval personnel's trespassing of Iranian territorial waters, the issue can be easily settled."
Iran's president called Britain "arrogant" Saturday for not apologizing, media in Iran reported.
British Foreign Secretary Margaret Beckett said Saturday that Britain has written to Iran to seek a peaceful resolution to the standoff.
Third letter released
On Friday, Iran released a third letter purportedly written by detained British sailor Faye Turney, in which she claimed to have been "sacrificed" by British and U.S. policies and urged both countries to withdraw their troops from Iraq. (Full story)
The letter, the authenticity of which CNN cannot independently determine, followed two previous letters said to be written by Turney and released separately this week. (Watch Turney say what happened when she was captured )
Friday's letter was released just hours after Turney appeared with two other Britons in new video aired by Arabic language network Al Alam. (Text of letters)
In the video, one of the 15 detained service personnel held in Iran confessed to "entering your waters without permission."
"On the 23rd of March 2007 in Iranian waters we trespassed without permission," said Nathan Thomas Summers. The third detainee in the video has not been identified. (Watch detained British sailor make his 'confession' )
Summers said the Britons were being treated well, as did the Turney letter.
CNN's Shirzad Bozorgmehr contributed to this report