Clashes between forces loyal to Yemeni President Ali Abdullah Saleh and student protesters and military forces protecting them in the capital Sana'a left at least 40 people dead Saturday, a day after the president unexpectedly returned to the country and called for a truce.

Government forces fired ammunition and mortar shells at a student protest camp overnight, causing many casualties. They fled in different directions to escape the attack, with some taking refuge inside the university campus.

A spokesman for opposition General Ali Mohsen al Ahmar said at least 11 soldiers loyal to him were killed after their position came under fire.

Forces loyal to President Saleh reportedly targeted the homes of tribal leader Sheikh Sadek al Ahmar and his supporters who no longer back Mr. Saleh.

Al Arabiya TV said that the six-nation Gulf Cooperation Council appealed to “all sides to observe a ceasefire” and to “stop using heavy artillery against unarmed protesters.”

Mr. Saleh called for an end to the fighting Friday, shortly after his return from Saudi Arabia where he was recovering from injuries sustained in a June attack on his presidential compound in Sana'a.

In a statement released by his office, Mr. Saleh said the solution to Yemen's mounting anti-government unrest was dialogue — not “guns and cannons.”

Protests against the embattled president have escalated over the past week, with clashes leaving more than 128 people dead.

In recent days, Yemen's political parties said they were working on a plan to force Mr. Saleh out of power.