Snipers killed up to 42 protesters among crowds that flocked to a sit-in at Sanaa University after Friday prayers. The opposition says at least seven snipers were caught carrying government identity cards, a charge the government has denied.
"Mr. Alsaidi has sent his resignation to the president's office and the Foreign Ministry," a Yemeni Foreign Ministry official said on Sunday, and added that it was in protest at the Friday violence.
Defying a crackdown after the government called a state of emergency, the opposition has vowed to keep up its pressure to end Saleh's 32-year rule in the poor Arabian Peninsula state, a neighbour of Saudi Arabia and a U.S. ally against al Qaeda.
Two prominent members of Yemen's ruling party resigned on Saturday in protest against the violence, and on Friday the tourism minister and the head of the party's foreign affairs committee quit.
Nasr Taha Mustafa, head of the state news agency and a leading ruling party member, said on Saturday he was giving up both his post and the party.
Another party member, Mohamed Saleh Qara'a, told Reuters on Saturday he had also quit because of the "completely unacceptable" violence.
Inspired by mass protests that toppled the leaders of Tunisia and Egypt, demonstrators have been demanding for weeks that veteran president Saleh step down. More than 70 people have been killed.
Saleh has blamed gunmen among the protesters for the killings.