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More than 20 people, including five soldiers, have been killed by state security services or regime-linked militias since Saturday, activists said on Tuesday, the fourth day of an ongoing sit-in outside of the army headquarters in Sudan's capital Khartoum.

Thousands of protesters began streaming to the Army General Command on Saturday to call for the armed forces to join them in calling for an end to the 30-year rule of President Omar al-Bashir and the formation of a transitional government.

The mass sit-in has faced repeat attacks from the security services and regime-linked militias, often in the dead of night.

Such attacks prompted lower-ranking members of the armed forces to disobey orders and step in to protect their fellow citizens.

Activists on social media reported soldiers shooting at and driving the security forces away, as well as members of the armed forces apprehending security officers and even a sniper.

In speeches made at the sit-in on Tuesday and broadcast on social media, soldiers explicitly said they had disobeyed orders to protect protesters.

— Sudanese Translators for Change STC (@SudaneseTc) " style="color:#fff;" class="twitter-post-link" target="_blank">Twitter Post The massive sit-in taking place at the Army Headquarters in Khartoum. — Sudanese Translators for Change STC (@SudaneseTc)

In an interview with °®Âþµº's Arabic service, army spokesman Brigadier General Ahmad Khalifa al-Shami confirmed that junior military officers had disobeyed orders by intervening in the sit-in.

"The small percentage of dissidents are junior officers who do not have enough experience," he said.

"They are from the same generation of young people now on the street."

He added: "From now on, the army will not allow matters to spiral out of control and it will not tolerate chaos."

As of Tuesday afternoon, 22 people have been killed since Saturday, including 5 soldiers, the Central Committee of Sudanese Doctors (CCSD) said.

At least eight of those people were killed on Tuesday alone. 

In addition, 153 people were reported wounded by the anti-Bashir doctors association, which has put out repeated calls for medical professionals to help treat wounded protesters in low-staffed hospitals.

"Clearly a lot of people have died," said spokesperson for the UN High Commissioner for Human Rights Ravina Shamdasani on Tuesday, explaining that it has been difficult for the UN to verify the exact number of people who have died since mid-December, when protests began.

It had also been difficult to determine how protesters have died or who exactly killed them, she said.

The UN High Commissioner Michelle Bachelet said 70 people had died in Sudanese demonstrations.

"The time has come for the Sudanese authorities to respond to these popular demands in a serious and credible way,"  the US, UK and Norway said in a joint statement in Khartoum on Tuesday.

"The Sudanese authorities must now respond and deliver a credible plan for this political transition."