"The shell had hit the entrance of the mosque, not the mosque itself, hitting people who were stood outside the mosque after their evening prayer, standing talking to one another, shaking hands. They did not deserve it," he said.
The number of people around the mosque was higher than usual, he said.
"People are now combining their evening and night prayers because of fear of Israeli strikes. It was as they were leaving the mosque when they were hit," he said.
Sobih, who has been an ambulance worker for more than a decade, said that he saw no evidence of any Hamas fighters among the injured.
"They were civilians, and some were children. Some neighbours of the mosque were helping the casualties, with blankets and bringing them to the hospital, some neighbouring houses were also damaged by the rockets. No one expected that Israel would fire here - it is very crowded and so close to the hospital."
Kamal Adwan Hospital, which is about 400 metres from the mosque, recorded that 12 people had been killed and 30 injured, 12 of them seriously. Six of the dead victims are believed to be under 18 years old.
The mosque is located in a densely built area of apartment blocks and low-level housing. Reports claimed that there were more than 200 people inside the mosque when the Israeli shells struck.
It was not clear whether any of the dead were Hamas fighters. The Israelis have claimed it was suspected that the mosque has been housing militants - it is named after a founder of Hamas who was killed by the Israelis in 2004. However, locals have claimed that radicals no longer control the mosque.
The attack will provoke further questions about the Israeli Defence Force's tactics as they search out Hamas's military hardware and hardline activists.
The Israeli military has destroyed several mosques during its week-long offensive in Gaza, claiming that Hamas uses them to store weapons. Artillery fire is less accurate than attacks from the air using precision-guided munitions, raising the possibility of a higher number of civilian casualties.
Ayman Mohyeldin, al-Jazeera's correspondent in Gaza, said that attacks on mosques could galvanise the Arab world into taking action against Israel. "This is proof that civilians are caught up in these attacks."
Mohyeldin said doctors in Gaza were being overwhelmed by the number of casualties as a result of the Israeli offensive and that hospitals were near a state of collapse due to a lack of medicines and blood stocks.
An artillery shell hit another building in Beit Lahiya, killing two people and wounding five, said members of the family living there. Ambulances could not immediately reach them because of the resulting fire, they said.
Before yesterday's offensive, Israel's bombing campaign had killed more than 430 Palestinians and left hundreds injured. The United Nations has said that at least a quarter of the dead were civilians. Much of Gaza's public infrastructure has been destroyed and the territory is in a state of emergency after eight days of bombing, averaging one blast every 20 minutes.
Israel's deputy chief of staff, Brigadier General Dan Harel, said that after Israel's operation was finished "no Hamas building will be left standing in Gaza". He added: "We are hitting government buildings, production factories, security wings and more."
Among the targets so far have been the Gaza Interior Ministry, police stations, television stations, prisons and a five-storey building in the women's wing of the Islamic University. Humanitarian organisations have criticised the Israelis for bombing a number of schools and a hospital.