"It was like a series of earthquakes," a shaken resident told The Associated Press, asking that his name not be used out of fear for his personal safety. "It was terrifying, terrifying."
The Syrian government said the bombings killed 34 people and injured 122 — although death tolls have been difficult to verify. The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said at least 40 people were killed.
The state-run Ikhbariya TV channel showed massive damage around Saadallah al-Jabri Square, which also houses a famous hotel and a coffee shop that had been popular with regime forces. One building appeared to have been leveled and the facade of another was torn away.
The station broadcast video of several bodies, including one being pulled from a collapsed building. Rescuers stood atop piles of concrete and debris, frantically trying to pull out survivors.
Activists could not reach the area, which is controlled by security forces and sealed off with checkpoints.
The uprising against Assad began in March 2011 and gradually became a bloody civil war. The conflict has killed more than 30,000 people, activists say, and has devastated entire neighborhoods in Syria's main cities, including Aleppo.
Syria's government has always blamed the uprising on what it calls foreign terrorists, despite months of peaceful protests that turned violent after repeated attacks by security forces. The transformation of the conflict into an open war has given an opportunity to foreign fighters and extremists, analysts say.
The Syrian opposition denies any links to terrorists, but a Sunni extremist group called Jabhat al-Nusra, or Victory Front, has claimed responsibility for bombings in the past.
After Wednesday's blasts in Aleppo, regime forces unleashed shelling on rebel-held areas and fired machine guns from aircraft, according to an Associated Press journalist in the city, Syria's largest with a population of 3 million.
At least 15 people wounded by shelling arrived with serious injuries at the city's Shifa Hospital. All but one were civilians. Three bodies — an old man, a woman and a middle school-age boy — also were taken to the hospital.
Rebel fighters, many with only light weapons, advanced slowly, moving building by building. The heavier weapons, such as rocket-propelled-grenade launchers, were sent to the front lines to prevent the regime from retaking areas seized by rebels in the past two months.
Wednesday's attacks were the latest turn in the deadly — and increasingly chaotic — fight for control of Aleppo, one of the world's oldest continuously inhabited cities.
Long free of the violence that has engulfed much of the rest of the country in the first year of the uprising, Aleppo was struck by two suicide car bombings at security compounds in February, killing 28 people. Such attacks targeting security agencies and soldiers have become common in Syria, particularly in the capital of Damascus.
In the past two months, Aleppo has become a key battleground. The opposition launched an offensive on the city in July, and large swaths have been shattered.
Rebels last week announced a new push to capture Aleppo, which would be a major strategic prize and give the victor new momentum. It also would provide the opposition with a base and easy logistical supply lines with Turkey to the north that would allow them to carry out their fight against the regime in the rest of the country.
Aleppo-based activist Mohammad Saeed said Wednesday's blasts went off minutes apart and appeared to be car bombs and were followed by clashes and heavy gunfire.
Syrian state TV said three suicide bombers detonated cars packed with explosives in Saadallah al-Jabri Square, near an officers' club. The square holds symbolic importance for residents because it is named after a Syrian independence fighter who resisted French occupation.
Activists and Syrian state media said a fourth car bomb went off a few hundred meters (yards) away in the Bab Jnein area near the Old City. It was not immediately clear how many casualties there were from that blast.
The Syrian Observatory for Human Rights said mortars also targeted the nearby political security department around the same time of the bombings.
Syria's Interior Ministry vowed to "track down the perpetrators anywhere." The speaker of the Syrian parliament, Mohammad Jihad al-Lahham, told the assembly that he condemns "the countries that conspire against Syria and stand behind the terrorists."
Torchia reported from Istanbul. Associated Press writers Suzan Fraser in Ankara, Turkey; Zeina Karam in Beirut; Manu Brabo in Aleppo, Syria; Albert Aji in Damascus, Syria; Don Melvin in Brussels; and Lolita Baldor and Bradley Klapper in Washington contributed to this report.