British Prime Minister Gordon Brown has welcomed President Barack Obama as a "new chapter in both American history and the world's history" on the day of his inauguration in Washington DC.
But green and human rights campaigner Peter Tatchell says that while the US can have a black head of state, the reality is that Britain cannot - because Black and Asian Britons are barred by the current system of monarchy.
The PM praised Mr Obama as a "man of great vision and moral purpose" who would tackle the global financial crisis and other problems such as the environment.
Speaking from Downing Street, Brown said: "The whole world is watching the inauguration of President Obama, witnessing a new chapter in both American history and the world's history.
He continued: "He's not only the first black American president but he sets out with the determination to solve the world's problems."
Mr Brown said he had met Mr Obama in London and Washington, and spoken to him by telephone on a number of occasions.
"He is a man of great vision, he is a man of great determination and energy, and he is a man of great moral purpose," he said.
The Prime Minister added that Obama was clear that these were "extraordinary times" with "unprecedented challenges".
The new president had already shown "great courage" and "boldness" in setting out plans to combat the financial crisis, he declared.
However, activist Peter Tatchell said the key issue was facing up to why having a Black or Asian person as head of state could not happen in Britain.
“As the United States celebrates the inauguration of its first black President, Barack Obama, the race barrier to highest office remains firmly intact in the UK. For the foreseeable future, non-white people will be ineligible," he declared.
He continued: “Under our constitutional system, the head of state is the monarch – at present Queen Elizabeth II. The position of monarch and head of state is inherited, through the Windsor family line. The Windsor’s are white and only their descendants are eligible to be King or Queen; only their first-born can be the British head of state."
“The system of monarchy is, by default, racist. Although it was not devised with racist intent, it is racist in effect,” said Tatchell.
Black and Asian people in Britain have been celebrating Obama's ascendancy. There was a large rally to watch the inauguration in Haringey, London, among other places.