* Twin explosions at Friday prayers in Sunni stronghold
* Car bomb last week killed 24 in Beirut Shi'ite district
* Government officials urge calm, warn of sectarian strife (Adds accusation of Syrian involvement)
By Oliver Holmes and Nazih Siddiq
TRIPOLI, Lebanon, Aug 23 (Reuters) - Bombs hit two mosques in the northern Lebanese city of Tripoli on Friday, killing at least 42 people and wounding hundreds, intensifying sectarian strife that has spilled over from the civil war in neighbouring Syria.
The apparently coordinated blasts - the biggest and deadliest in Tripoli since the end of Lebanon's own civil war - struck as locals were finishing Friday prayers in the largely Sunni Muslim city. Lebanese officials appealed for calm.
The explosions in Tripoli, 70 km (40 miles) from the capital Beirut, came a week after a huge car bomb killed at least 24 people in a part of Beirut controlled by the Shi'ite Muslim militant movement Hezbollah.
A recent resurgence of sectarian violence in Lebanon has been stoked by the conflagration in Syria, where President Bashar al-Assad is fighting a largely Sunni-led rebellion. Both Hezbollah and radical Sunni groups in Lebanon have sent fighters over the border to support opposing sides in Syria.
Medical and security sources said the death toll from Friday's blasts in Tripoli had risen to 42 by late afternoon. Hundreds more were wounded, they said.
The first explosion hit the Taqwa mosque, frequented by hardline Sunni Islamists, and killed at least 14 people there, according to accounts earlier in the day.
Further deaths were reported from a second blast a few minutes later outside the al-Salam mosque, which the Interior Ministry said was hit by a car laden with 100 kg (220 pounds) of explosives.
A Reuters reporter at the scene said the blast left a huge crater and the floors of the mosque were covered in blood. A 50-metre (160-foot) stretch of the road was charred black and the twisted remains of cars littered the area.
"We were just bowing down to pray for the second time and the bomb went off. The air cleared, and I looked around me and saw bodies," said Samir Jadool, 39.
Lebanon's Red Cross said more than 500 people were wounded in the two explosions. Television footage showed people running through the streets, some of them carrying bloodied victims.
Near the Taqwa mosque blast site, angry men toting AK-47 assault rifles took to the streets and fired in the air while other men threw rocks at Lebanese soldiers nearby.
"Back off," said one militant, when journalists approached the scene. Soldiers peeked out at the mosque from a nearby base but did not approach it.
U.N. Secretary-General Ban Ki-moon condemned the bombings and called on all Lebanese people to "exercise restraint, to remain united, and to support their state institutions, particularly the security forces".
"BEGINNING OF THE STORM"
Witnesses at the scene of the blasts said anger was rising among locals, who were shouting out accusations that Assad's government or Hezbollah were behind the attack.
"This is the work of a criminal," seethed Jadool, who held a bloodied bandage against his head, the result of flying debris from the blast at the al-Salam mosque.
Video obtained by local news channel LBC showed the moment of the explosion at al-Salam mosque. The blast ripped through a wall of the mosque, showering clouds of dust on people sitting on prayer mats and sending dozens running out of the building.
Lebanese officials called for calm as tensions rose in Tripoli, a Mediterranean port that has seen some of the worst Syria spillover violence. Sunni gunmen have sporadically clashed with fighters from the city's minority Alawite sect, an offshoot of Shi'ite Islam to which the Assad family belongs.
Former internal security chief Ashraf Rifi, whose home was damaged by the second blast, warned Lebanon was facing a growing threat. "We are still in the beginning of the storm and we must remain aware and try to protect this nation," he said, speaking outside his home. "This storm has become a huge, grave danger."
Officials in Tripoli called on the government in a joint statement to step up security in the city. Mohammed Kabara, a member of parliament who read out the statement, accused the Syrian government of carrying out the Beirut and Tripoli bombings to create strife in Lebanon.
Prominent Salafist sheik, Dai al-Islam Shahhal, also blamed the Syrian government for the Tripoli bombs and said it was "pure terrorism".
Hezbollah released a statement condemning the Tripoli blasts and expressing solidarity with the victims, saying they were targets of efforts to fan more violence in Lebanon.
"We consider this the completion of an effort to plunge Lebanon into chaos and destruction," the statement said.
Hezbollah's political opponents called on the group to withdraw its forces from Syria in response to Friday's attack.
Lebanese Defence Minister Fayez Ghosn warned against being dragged into deeper sectarian bloodshed. "We are calling for calm and vigilance, because the aim of this (the bombings) is to stoke strife between sects," he told LBC.
Salem al-Rafei, chief cleric of the Taqwa mosque, is a staunch supporter of Syrian Sunni rebels as well as Lebanese Sunni militants who have joined the anti-Assad battle in Syria. (Additional reporting by Michelle Nichols at the United Nations; Writing by Erika Solomon and Mariam Karouny; Editing by Pravin Char)