The death toll amongst protesters staging a sit-in outside Sudan's military headquarters has risen to 60, a doctors' group associated with the demonstrations said on Wednesday. The deaths marked the deadliest day of violence in the country since President Omar al-Bashir was forced from power in April.

Streets in central Khartoum remained on lockdown on Wednesday, and military vehicles were patrolling its twin city Omdurman, firing shots into the air, Deutsche Welle reports.

Tuesday's violence came as negotiations broke down between the Transitional Military Council (TMC) and the protesters over how the country should transition to democracy following the end of Bashir's decades-long rule.

The two had first agreed to plan to hold elections in three years. In the interim, a caretaker government with a president rotating between the TMC and civilians would have been put in place.

However, the military then announced it would hold elections in nine months, and appeared to walk back promises that civilians could be part of the interim government.

Omar al-Degair, a leader of the open-ended civil disobedience movement, said that they had "agreed with a large majority to this agreement presented by the TMC and we were meant to hold an expanded meeting last night to reach a final agreement," before the TMC changed its mind.

The military council has since announced it was nixing any agreement it had made.

The Sudanese Professional Association (SPA), one of the main protest organizers, condemned the deaths of demonstrators at the hands of security forces as a "massacre" and called for an international committee to investigate the deaths.