Nigerian armed groups must stop attacking civilians, Amnesty International has said, after as many as 30 people were killed in a bomb attack blamed on the religious sect Boko Haram.

Motorcyclists hurled bombs into a beer garden killing up to 30 people in Maiduguri, Borno State, in the northeast of the country on Sunday, before shooting into the crowd. Several people were injured in a fresh bombing on Monday, also believed to have been carried out by Boko Haram.

The attacks are the latest in a series of bombings targeting civilians blamed on Boko Haram, an armed group which seeks to establish Sharia law in parts of Nigeria.

“These killings are senseless and outrageous. Direct attacks on civilians are prohibited under international law and show a complete disregard for the right to life,” said Tawanda Hondora, Amnesty International’s Deputy Director for Africa.

“Boko Haram must stop its reign of terror in the country. No cause can justify the deliberate targeting of civilians.”

Boko Haram has claimed responsibility for a spate of bombings across the country since mid 2010. In the past several weeks, dozens of people have been killed in the attacks. On 16 June, a church was bombed, reportedly by Boko Haram, killing three children. The group has also attacked Muslim clerics who have criticised them. Previously, the group tended to target police and other government authorities.

In response, the Nigerian security forces have carried out mass arrests, tortured suspects and detained people without charge or trial for lengthy periods.

Amnesty International is calling on the Nigerian government to step up the protection of residents in the Northeast, and ensure the rule of law.

“The Nigerian government can only ensure safety by investing heavily in reforming the criminal justice system, so that the perpetrators of these attacks and other human rights abuses, can be properly investigated, arrested and prosecuted in fair trials without recourse to the death penalty.”

The Nigerian criminal justice system is dysfunctional at best and subjects thousands to human rights violations, including torture, enforced disappearances, extrajudicial executions, and unfair trials. Suspected members of Boko Haram have been rounded up in previous sweeps, but have not been tried, due in part to the lack of proper police work to gather evidence.