Human Rights Watch said: "In response to a nationwide strike protesting [against] increases in the prices of rice and fuel, Guinea's security forces committed murder, rape, assault and theft against demonstrators and bystanders alike.
"Security forces fired directly into crowds of unarmed demonstrators. Police also beat other protesters with truncheons and rifle butts."
There were witnesses to 13 killings in the capital Conakry and the regional capital Labe in eastern Guinea, it said.
The group quoted the testimony of a 19-year-old woman who described being raped in her home by a policeman, and of others who suffered or witnessed shootings and beatings in the streets.
The violence started when students took to the streets in several towns to protest against the government after their teachers joined the strike, disrupting their exams. In some places, students threw rocks at security forces and damaged government property.
As the demonstrations turned violent on the fourth day of the general strike, police pursued protesters into surrounding neighbourhoods where they beat residents who had not taken part in the protests and robbed people at gunpoint, the rights group said.
Peter Takirambudde, Africa director for HRW, said: "The Guinean government cannot allow its security forces to get away with killing unarmed protesters and attacking bystanders.
"The lethal use of force against protesters was wholly disproportionate and inappropriate in the circumstances.
"The Guinean government must demonstrate that it is serious about protecting the lives of all Guineans by quickly investigating these abuses."
The director of national security gave the official death toll from the violence as 11, but hospital sources and witnesses in Guinea told AFP that 18 people had been killed and 83 injured across the country.
HRW said on Thursday that local civil society groups said 21 people had been shot.
The strike ended on June 16 when the government agreed to salary increases and talks on petrol prices.
Guinea: Security forces respond to protests with killings - Human Rights Watch, July 6, 2006