"No one knows what the meaning of four years without a dad," said Anas who walked to Blair's Number 10 Downing Street with his nine-year-old brother Mohammed seeking information about their father, reported Agence France-Presse (AFP).
"No one knows how my life has changed after my dad was kidnapped from my life only because he didn't have British passport," wrote the ten-year-old child.
The father, Jamil, was arrested with a friend during a business trip to Gambia in 2002 on suspicion of links to terrorist groups.
They were handed over to US security officers and transferred to the notorious detention camp in Cuba.
The child's letter to Blair is part of worldwide protests to mark five years since Guantanamo was opened on January 11, 2002.
Washington has been holding hundreds of detainees at the notorious detention facility, mostly arrested in Afghanistan after the toppling of Taliban following the 9/11 attacks. Only ten of them have been indicted for charges.
Guantanamo's buildings hide behind multiple rows of 12-foot chain-link fences covered in green tarpaulins and topped with tight spirals of barbed wire.
Old wooden and newer steel watchtowers dot the perimeter.
Anas, in his fourth letter to Blair, one each year for the four his father has been held, asked the prime minister if he really cares for the fate of his father.
"Your children spend Christmas with you but me and my brothers and sisters have spent `Eid alone without our dad for three years," wrote the kid.
"What do you think about that? I hope you answer me this time."
Jamil, a Jordanian, and his friend, Bisher al-Rawi, an Iraqi, are not British citizens but have the right to reside in Britain.
He is one of eight British residents believed to be held at Guantanamo.
All British citizens were freed from the detention center by September 2004, but London insists it has no power to intervene on behalf of foreign nationals, even if they have long been resident in Britain.
"What are 350 people still doing there?" asked Asif Iqbal, a Briton of Pakistani descent who was released from the detention camp two years ago after being held for about three years.
"We believe they are innocent until proven guilty," said the 25-year-old man who used to consider the US "a beacon of hope" and now considers it "worse than any communist country."
Iqbal is one of hundreds of relatives and activists who traveled to Cuba with the hope of having a glimpse of their beloved held in the notorious camp.
"I'm very excited to be so close to my son," said a tearful Zohra Zewawi, standing by a monument to the revolution in the Cuban city of Guantanamo.
"My big wish is to release him, see him and hug him; and for all the others to be freed."
Day of Shame
"Guantanamo has come to symbolize the hollowness of the US government's promise that respect for human dignity," Khan said.
Human rights groups said that the US practices at the Guantanamo Bay had weakened human rights and the rule of law around the world.
"January 11, 2007, is a national day of shame," Britain's Stop the War Coalition said in a statement.
"Guantanamo has come to symbolize the hollowness of the US government's promise that respect for human dignity and the rule of law would lie at the heart of its response to the attacks of 11 September 2001," agreed Irene Khan, Secretary General of Amnesty International.
"Torture, humiliation, discrimination, bypassing of the courts and disregard for treaty obligations, with almost total impunity, are all now among the entries in the Guantanamo logbook."
The London-based human rights group, which has planned global vigils to mark the anniversary in many world countries, called for shutting down the notorious US detention camp.
"The US government must end this travesty of justice," said Khan.
"With every passing day, the cruelty of this indefinite detention regime ratchets up another notch."
Amnesty insists Guantanamo has become a "symbol of abuse and represents a system of detention that is betraying the best US values and undermines international standards."
The international rights watchdog once likened it to gulag prisons, the Soviet detention centers notorious for torturing political prisoners and suspects.
Click to read the letter