Three suicide car bombs and a mortar barrage ripped through a government-controlled district of central Aleppo housing a military officers' club on Wednesday, killing 48 people according to activists.
The coordinated attacks hit just days after rebels launched an offensive against President Bashar Assad's forces in Syria's biggest city, leading to heavy fighting and a fire which gutted a large part of its medieval covered market.
The state news agency SANA said suicide bombers detonated two explosive-laden cars in the main square, Saadallah al-Jabiri, which is lined on its eastern flank by the military club, two hotels and a telecoms office.
The explosions reduced at least one building to a flattened wreck of twisted concrete and metal, and were followed by a volley of mortar bombs into the square and attempted suicide bombings by three rebels carrying explosives, it said.
Another bomb blew up a few hundred meters (yards) away on the edge of the Old City, where rebels have been battling Assad's forces.
State television showed three dead men disguised as soldiers in army fatigues who it said were shot by security forces before they could detonate explosive-packed belts they were wearing. One appeared to have a trigger device strapped to his wrist.
Another pro-Assad station, al-Ikhbariya TV, broadcast footage of four dead men, including one dust-covered body being pulled from the rubble of a collapsed building and loaded onto the back of a pickup truck.
The facades of many buildings overlooking the square were ripped off and a deep crater was gouged in the road. The Syrian Observatory for Human Rights said 48 people were killed, mostly from the security forces, while SANA put the death toll at 31.
Wednesday's attacks in Aleppo followed last week's bombing of the military staff headquarters in Damascus, another strike by Assad's outgunned opponents against bulwarks of his power.
In July, rebels killed four of Assad's senior security officials including Assad's brother-in-law, the defense minister and a general in a Damascus bombing which coincided with a rebel offensive in the capital.
Government forces have since pushed rebel fighers back to the outskirts of Damascus. But they have lost control of swathes of northern Syria as well as several border crossings with Turkey and Iraq and failed to push the fighters out of Aleppo.
A pro-Assad Lebanese paper said on Tuesday that Assad was visiting the city to take a first-hand look at the fighting and had ordered 30,000 more troops into the battle.
Many rich merchants and minority groups in Aleppo, fearful of instability, remained neutral while protests spread through Syria. But rebels from rural Aleppo swept into the city in July and still control districts in the east and south.
Opposition activists say 30,000 people have been killed across the country in the 18-month-old uprising, which has grown into a full-scale civil war with sectarian overtones and threatens to draw in regional Sunni Muslim and Shi'ite powers.
Sources in Lebanon said seven members of Lebanon's Shi'ite Muslim militant group Hezbollah, a close ally of the Syrian president, were killed inside Syria on Sunday in a rocket attack. Three were killed instantly while four others were wounded and died subsequently, they said.
The sources said the Hezbollah fighters were operating in the border area, monitoring the flow of weapons into Syria from Lebanon.
Hezbollah's website and television station said the group held funerals this week for two fighters killed while performing "jihadi duties", but gave no further details.
Hezbollah has given strong public political support to its ally in Damascus but has not confirmed a military presence on the ground in Syria - wary of inflaming sectarian tensions in Lebanon, where many Sunni Muslims support the anti-Assad rebels.
The mainly Sunni rebels are supported by Sunni powers including Turkey, Saudi Arabia and Qatar, and have attracted Islamist fighters from across the Middle East to their cause.
Assad, from the Alawite minority which is an offshoot of Shi'ite Islam, is backed by Iran and Russia.
Russian Deputy Foreign Minister Gennady Gatilov said on Tuesday NATO and world powers should not seek ways to intervene in the war or set up buffer zones between rebels and government forces.
He also called for restraint between NATO-member Turkey and Syria, after tensions flared when a mortar round fired from inside Syria struck the territory of Turkey. Ankara has threatened to respond if the strike were repeated.
Iran's President Mahmoud Ahmadinejad warned that hostilities in Syria could engulf the region and accused some Syrians of trying to use the conflict to settle scores with Tehran.
Ahmadinejad said that a national dialogue and elections - rather than war - were the only way to solve the Syrian crisis.
Efforts to address the conflict at the United Nations have been blocked by a standoff in the Security Council between Western powers seeking a tough stance against Assad and Russia and China, which fear a U.N. resolution against Syria would be the first step towards military intervention.
An Egyptian attempt to bring together Egypt, Turkey, Iran and Saudi Arabia to search for a regional solution to the crisis also appeared to be going nowhere after Saudi Arabia stayed away for a second time from a meeting of the four countries.
Additional reporting by Dominic Evans and Laila Bassam in Beirut, Editing by Samia Nakhoul and Angus MacSwan