Amnesty International says it has received "credible reports" that Gambia executed nine death row prisoners on Thursday.
"More persons are under threat of imminent executions in the coming days," Amnesty International said.
President Yahya Jammeh had vowed to kill all 47 death row inmates by next month, in a national speech to mark the Muslim festival of Eid.
The last official execution in Gambia took place in 1985.
The African Union called on Mr Jammeh to renounce his plans after he made the announcement on Sunday.
But according to Amnesty International, nine people, including one woman, were removed from their prison cells and executed on Thursday night.
Three of those reportedly executed had been sentenced for treason, the group said in a statement.
"The decision of the Gambian President Yahya Jammeh to execute nine prisoners after more than a quarter of a century without execution would be a giant leap backwards," said Paule Rigaud, Amnesty International's deputy director for the Africa region.
She added that many of the death row inmates were political prisoners or have faced unfair trials.
A Gambian security source told AFP news agency that all 47 death row prisoners had been "transferred to one place".
Referring to President Jammeh, the source said: "The man is determined to execute the prisoners and he will do so."
The death penalty was abolished when former President Dawda Jawara was in power but reinstated in 1995 shortly after Mr Jammeh seized power in a military coup.
"By the middle of next month, all the death sentences would have been carried out to the letter; there is no way my government will allow 99% of the population to be held to ransom by criminals," President Jammeh said in an speech on Sunday, which was broadcast on national television the next day.
In response, Benin's President Thomas Boni Yayi, who is the current chair of the African Union, sent his foreign minister to Gambia.
"After having learned of the imminent execution of a number of prisoners sentenced to death, President Yayi, who is very concerned, wished that President Yahya Jammeh not carry out such a decision," Beninois Foreign Minister Nassirou Bako Arifari told BBC Afrique.
Mr Jammeh's human rights record has often been criticised by international organisations, with particular concerns over press freedom.
Last year, after winning a fourth term in office in widely criticised polls he told the BBC that his critics could "go to hell" because he feared "only Allah".
The tiny West African state is a popular tourist destination.