The opposition says the November 26 election was rigged, and the country's electoral authority has yet to declare a winner even though the final count gave incumbent Orlando Hernandez a narrow edge over challenger Salvador Nasralla.
Thousands of protesters took to the streets on Friday to condemn the prospect of another term for Hernandez, a day after thousands of his supporters demonstrated to back his claim of victory.
"At least 14 people died, most of them from bullet wounds, and dozens were injured in the context of largely peaceful demonstrations since the elections," Amnesty said in a statement, citing human rights defenders and public servants.
Security forces personnel have also tear gassed and arrested protesters, the rights group said.
"Halting all use of illegitimate or excessive force against protesters by security forces, ending arbitrary detentions, and investigating all instances of human rights violations would be a good start to undo some of the many wrongs we have documented in recent days," said Erika Guevara-Rosas, Amnesty's Americas director.
International observers have expressed reservations over the poll, which the Organization for American States said was marred by irregularities that cloud the results.
Nasralla has refused to recognize the current results, while Hernandez has claimed victory.
