Mauritian Prime Minister Pravind Jugnauth has labelled the British and Americans "hypocrites" and "champions of double talk" for the way they have behaved over the Chagos Islands.
Last year, a UN court ruled that the UK should end its control of the Indian Ocean archipelago, which includes a US military base.
Mauritius says it was forced to trade the islands in 1965 for independence.
The UK has said it does not recognise Mauritius' claim to sovereignty.
Between 1968 and 1974, the UK forcibly removed thousands of Chagossians from their homelands and sent them more than 1,600km (1,000 miles) away to Mauritius and the Seychelles, where they faced extreme poverty and discrimination.
On Sunday in the Mauritian capital, Port Louis, in front of a crowd composed mainly of Chagossians and their descendants, Mr Jugnauth said the UK and US lectured countries "to respect human rights, but they are champions of double talk."
"They are hypocrites. Shame on them when they talk about human rights and respect," he added.
With the backing of the International Court of Justice (ICJ), Mauritius no longer wants to be pushed around by its former colonial power, reports the BBC's Yasine Mohabuth from Port Louis.
In a statement last year the UK Foreign and Commonwealth Office said that the UK "has no doubt as to our sovereignty over the British Indian Ocean Territory (BIOT), which has been under continuous British sovereignty since 1814".
"Mauritius has never held sovereignty over the BIOT and the UK does not recognise its claim."
But at the ICJ, Judge Abdulqawi Ahmed Yusuf described the UK's administration of the Chagos Islands as "an unlawful act of continuing character".
In an advisory opinion in February 2019 the ICJ said that the archipelago should be handed over to Mauritius in order to complete its "decolonisation".
'A just fight'
Then three months later, the UN General Assembly voted overwhelmingly in favour of the Chagos Islands being returned - with 116 states backing the move and only six against.
But the UK did not act describing the ICJ ruling as an "advisory opinion, not a judgment".
On Sunday, Mr Jugnauth said: "Our fight is just. The majority of countries support us... we will not retreat," he added.
Last year, he described the UK as an "illegal occupier".
The Chagos Archipelago was separated from Mauritius in 1965, when Mauritius was still a British colony. Britain purchased it for £3m - creating the BIOT.
'Crime against humanity'
Mauritius says it was forced to give it up in exchange for independence, which it gained in 1968.
The Chagossians were evicted and the UK then invited the US to build a military base on Diego Garcia.
US planes have been sent from the base to carrying out bombing raids in Afghanistan and Iraq. The facility was also reportedly used as a "black site" by the CIA to interrogate terrorism suspects. In 2016, the lease for the base was extended until 2036.
The UK has repeatedly apologised for the forced evictions, which Mr Jugnauth has said were akin to a crime against humanity. It has also promised to hand the islands over to Mauritius when they are no longer needed for security purposes.
The UK government has said: "The defence facilities on the British Indian Ocean Territory help to protect people here in Britain and around the world from terrorist threats, organised crime and piracy."
In 2002, the British Overseas Territories Act granted British citizenship to resettled Chagossians born between 1969 and 1982. But the 13-year window has left some families divided.
Source: BBC News