A year after Sudan's former president was forced to step down amid mass anti-government protests, little has been done to bring justice to the families of those killed in the demonstrations, a rights group said on Friday.

Calling on the country's transitional government to prosecute crimes committed at the time, Human Rights Watch (HRW) said in a statement that security forces had used lethal and excessive force against protesters, killing dozens.

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The months-long protests - which began on December 19, 2018, over rising costs of bread and fuel - quickly transformed into calls to overthrow longtime leader Omar al-Bashir's government, and then called for a handover of military rule to a civilian-led government.

Al-Bashir stepped down on April 11, 2019, after nearly 30 years in power in Sudan.

Independent groups estimate the death toll was more than 100 between December 2018 and April 2019. According to Amnesty International, at least 77 people were killed during that period.

"Scores of protesters, including teenagers and children, paid with their lives to force al-Bashir out, but a year on, the families of those killed are still searching for justice," said Jehanne Henry, HRW's East Africa director.

"Sudanese authorities should step up their efforts to do right by these victims. Justice should not be denied or delayed," she added in the statement.

Deadliest episode

Violence against protesters also continued after the overthrow of al-Bashir.

Thousands of Sudanese protesters camped outside the army headquarters in the capital, Khartoum, demanding al-Bashir's removal and kept up their sit-in even after his departure to protest against the military council that took over in his place.

On June 3, armed men in military fatigues moved in on the protest camp and dispersed thousands of demonstrators. In the ensuing days-long crackdown, many were killed and wounded.

Doctors linked to the protest movement have said at least 128 people were killed in the violence. Authorities gave a lower death toll of 87, and denied ordering the deadly dispersal.

In a scathing report released in March titled Chaos and Fire, the US-based NGO Physicians for Human Rights said the crackdown was a "massacre" that could have claimed up to 241 lives.

After documenting the events of June 3, HRW concluded they could qualify as crimes against humanity.

Sudan's new authorities set up an independent commission to investigate the attack, but the team has yet to release its findings.

HRW said cases of violations against protesters in Sudan are still being handled in an "ad hoc manner".