The UK-based rights group said Guinean security forces fired live rounds against protesters during post-election unrest that has killed at least 10 people.
In a statement on Sunday, the United Kingdom-based rights group said witness statements and video analysis confirmed protesters were targeted.
Amnesty also condemned internet disruptions during the deadly violence.
President Alpha Conde, 82, won a controversial third presidential term – which requires confirmation by the Constitutional Court – with 59.49 percent of the votes, Guinea’s electoral commission said on Saturday.
But the country’s leading opposition politician Cellou Dalein Diallo has disputed the result and claimed victory himself a day after the contested October 18 poll.
He said he has evidence of fraud and plans to file a complaint with the Constitutional Court.
Diallo’s victory claim triggered clashes between his supporters and security forces across the country.
The opposition puts the one-week death toll at 27 people – a figure that cannot be independently verified at this time.
Amnesty said it was still analysing information but added that based on what it has already gathered, coupled with local news reports, “dozens of people might have been killed”.
In a video of recent unrest in the capital Conakry, the group said a security officer shot people at a very close range, “without any apparent threat to his life [and] in violation of international rules on the use of firearms by armed forces”.
Amnesty also analysed pictures taken in the northern Labe region – a Diallo stronghold – which showed bullet casings from AK-47 assault rifles.
It said Guinean security forces deployed in the region often carry such rifles, although the government denies this.
“Authorities must stop the use of firearms,” Fabien Offner, an Amnesty researcher, said in the statement.
“If criminal culpability is found, those suspected must be brought to justice in fair trials before civilian courts,” he said.
Guinea’s government was not immediately available for comment.
‘Attack on freedom of expression’
Separately, Amnesty also criticised internet and phone-line cuts on Friday and Saturday – calling them a “frontal attack on freedom of expression”.
“This new standstill of various means of communications constitutes an attack on freedom of expression and an attempt to silence protesters, human rights defenders, journalists, and bloggers,” said Offner.
“The authorities must immediately lift the suspension of Guinéematin.com news website and the restrictions on access to the internet and social media so that everyone can freely express himself and journalists can do their job.”
A former opposition activist, Conde became Guinea’s first democratically-elected president in 2010 and won re-election in 2015.
Critics accuse him of drifting towards authoritarianism, however.
In March, the octogenarian president pushed through a new constitution he said would modernise the country.
It also allowed him to bypass a two-term presidential limit, which provoked mass protests from October 2019, during which security forces have killed dozens of people.
Before assuming office, Conde had been a long-standing opposition figure who was jailed and exiled for his views against Guinea’s military government.
Diallo is a former prime minister who also finished runner-up to Conde in the 2010 and 2015 elections.