(CNN) -- Nearly 40 people were killed in Yemen's capital on Saturday as protesters took to the streets and government Republican Guards clashed with Yemeni soldiers who support the opposition.
At least 26 protesters were killed when security forces fired at them and rockets landed in Sanaa's Change Square, according to medical staffers there. Change Square is the epicenter of the opposition movement in the capital. At least 52 protesters were injured.
Also in Sanaa, 12 dissident soldiers were killed and 112 others were injured when the guards attacked the 1st Armored Division, an army unit that defected, a spokesman for the defectors said.
Powerful explosions have been heard during clashes between the guards and the dissident soldiers.
"More than 93 rockets bombarded the 1st Division and many of the injured are in critical condition," said Abdul Ghani al-Shamiri, a spokesman for Gen. Ali Mohsen, a prominent defector who leads the division.
Republican Guards attacked a location in another city, Taiz, a medic there said. An eyewitness reported that security forces shot and killed one person Saturday night as he walked along a city street.
The clashes come a day after Yemeni President Ali Abdullah Saleh returned home after three months in Saudi Arabia, where he was treated for injuries he suffered in an assassination attempt in June. Saleh is unpopular in many quarters of Yemen and his opponents want him to leave office.
On Saturday, the U.N. Security Council issued a statement expressing its "grave concern at the continued serious deterioration of the economic and humanitarian situation in Yemen." It urged "all sides ... to reject violence" and "move forward urgently in an inclusive, orderly and Yemeni-led process of political transition."
U.S. State Department spokeswoman Victoria Nuland earlier in the day issued a statement "expressing deep concern about the current situation in Yemen" and again urging Saleh "to initiate a full transfer of power without delay and arrange for presidential elections" by year's end.
"Too many Yemenis have lost their lives, and each day that passes without a peaceful and orderly transition is another day that the Yemeni people are forced to live in an unstable environment that threatens their security and livelihood," she said.