Clashes between pro- and anti-government forces have left at least 40 people dead in Yemen on Saturday, a day after President Ali Abdullah Saleh unexpectedly returned to the country and called for a truce.

Saturday's unrest erupted in the capital, Sana'a, where government security forces have been battling opponents on several fronts.

Activists say forces have clashed with troops supporting tribal sheikhs who no longer back Mr. Saleh. They also say security forces have fired shots at demonstrators who have been camping out in a protest camp known as Change Square.

Mr. Saleh called for an end to the fighting shortly after he returned from Saudi Arabia on Friday. He spent three months in the neighboring country 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.

Earlier this week, representatives of the six-nation Gulf Cooperation Council met with Vice President Abed Rabbo Mansour Hadi in Sana'a in an attempt to revive a long-stalled plan that calls for President Saleh to hand over power to a deputy.