The opposition had Polish, Chinese and Russian small arms, Amnesty said, escalating the weapons buildup of both sides.
In 2004, the U.N. attempted to stem the flow of weapons with an international embargo -- to no avail.
Escalation to armed conflict
In November 2010, Gbagbo lost in a presidential election, to rival Alassane Ouattara, but refused to cede power.
The government's weapons came out. Soldiers gunned down opposition protesters in the streets, and Ouattara and his loyalists started an armed campaign to oust Gbagbo.
Hundreds died; many more were wounded, and hundreds of thousands fled to neighboring countries. Many women and girls were raped, according to the report.
The United Nations intervened militarily and Ouattara's troops took power by force in April 2011.
Both sides committed atrocities, Amnesty said, including the summary killings of "men and boys who were singled out based on their ethnicity or political affiliation."
Widespread weapons problematic
Abuses still go on, partly because of the high number of weapons still in the country.
They "continue to fuel grave human rights abuses and violent crime in the country," Amnesty said.
Amnesty supports the U.N.'s efforts and expressed hope that the world's nations will agree on the Arms Trade Treaty.
Ban wants the treaty to cover all conventional weapons, "from small arms to tanks to combat aircraft."
The treaty has been in the works since 2006, when the U.N. General Assembly requested all countries to submit their views on a binding arms trade treaty. That year, the United States, under the George W. Bush administration, voted against the resolution that started the process. But the Obama administration later reversed that decision.
The U.N. estimates that in 2010, more than 27 million people were still internally displaced because of armed conflict.