SANAA, Yemen – In one of the bloodiest days of Yemen’s uprising, government troops backed by snipers and shelling attacked a square full of protesters Saturday and battled with pro-opposition forces in the capital, killing at least 40 people and littering the streets with bodies.

The violence signaled an accelerated attempt by President Ali Abdullah Saleh and his loyalists to crush their rivals after his return a day earlier from Saudi Arabia, where he received treatment for the past three months for wounds suffered in an assassination attempt.

One of Saleh’s top rivals — Maj. Gen. Ali Mohsen al-Ahmar — asked the U.S. and other regional powers to help rein him in. He warned that Saleh is pushing the country into civil war and compared him to the Roman emperor Nero, burning down his own city.

Al-Ahmar called Saleh a “sick, vengeful soul” who treats Yemen like his personal estate.

“With his return, Yemen is experiencing sweeping chaos and the harbingers of a crushing civil war which this ignorant man is determined to ignite,” said al-Ahmar, who was once a close ally of Saleh but joined the opposition along with the 1st Armored Division he commands.

Sanaa has become a city divided between rival gunmen, with barracks and roadblocks manned by men in different uniforms indicating their loyalties. Many residents took cover in basements because of the ongoing thuds of mortars during fighting that has killed at least 140 people in the past week.

The turmoil is a blow to U.S. efforts to find a stable transfer of power to ensure the continued fight against al-Qaida militants in Yemen, who Washington says constitute the most dangerous branch of the terror network.

Regime forces on Saturday pounded the protest camp in Sanaa’s Change Square, where thousands were massed, as they have been nearly daily since February in peaceful protests demanding the end of Saleh’s 33-year rule. Mortar shells blasted in the square, setting a number of tents on fire. Snipers on rooftops fired down methodically on protesters dashing for cover.