* Nasrallah redoubles commitment to Syria campaign after bomb
* Twenty-four killed and 335 wounded in Thursday bomb
* Minister says suicide bomber may have carried out attack (Recasts with Nasrallah speech)
By Stephen Kalin and Mariam Karouny
BEIRUT, Aug 16 (Reuters) - Hezbollah leader Sayyed Hassan Nasrallah accused radical Sunni Islamists on Friday of being behind a car bomb that killed 24 people in Beirut and vowed that the attack would redouble his group's commitment to its military campaign in Syria.
In a fiery speech to supporters, one day after the deadliest bombing in the capital since Lebanon's civil war ended two decades ago, Nasrallah raised the stakes by pledging to join the battle in Syria himself if needed.
Thursday's blast in the Shi'ite militant Hezbollah's south Beirut stronghold followed months of sectarian tension and violence in Lebanon fuelled in part by Hezbollah's intervention against Sunni Muslim rebels in Syria's civil war.
"It is most likely that a takfiri group was responsible for yesterday's explosion," Nasrallah said, referring to radical Sunni Muslim factions linked to al Qaeda, many of whom are fighting with Syrian rebels against President Bashar al-Assad.
"If you think by killing our women and children ... and destroying our neighbourhoods, we would retreat from the position we took (in Syria) you are wrong," he said in a combative speech broadcast by videolink from a secret location to his supporters.
"If we had 100 fighters in Syria, now they will be 200. If we had 1,000, they will be 2,000. If we had 5,000 they will be 10,000. If the battle with these takfiri terrorists requires that I and all Hezbollah should go to Syria, we will go."
Thursday's blast came a month after a car bomb wounded 50 people in the same district of the Lebanese capital - an attack that Nasrallah also blamed on takfiris, who consider all but the most radical Sunnis to be infidels whose blood can be spilt.
Defence Minister Fayez Ghosn said a Syrian man had been arrested for suspected involvement in the July bombing, underlining the extent to which Lebanon has become embroiled in its neighbour's conflict.
Lebanese Hezbollah fighters helped Assad's soldiers retake a strategic border town in June, while Sunni Muslims from Lebanon have joined the rebel ranks. The violence has spilled back into Lebanon, with bombings and street clashes in the Bekaa Valley and Mediterranean cities of Tripoli and Sidon.
POSSIBLE SUICIDE BOMBING
Thursday's explosion engulfed a busy street in flames, reviving memories of the destruction inflicted by Lebanon's civil war.
Interior Minister Marwan Charbel said investigators were checking CCTV footage taken in the moments before the explosion to see whether the van believed to have carried the bomb had been driven by a suicide bomber or detonated remotely.
Reporters who arrived at the scene minutes after the explosion saw a burnt-out car near the centre of the road, suggesting it was being driven when it blew up.
Among the dead were a family of five - a father, mother and their three daughters - who were killed in their car by the blast, which destroyed several vehicles and set fire to the lower floors of adjacent buildings, trapping residents.
Forensic investigators, emergency workers and security forces were still working at the site on Friday, amid burnt-out cars and charred facades of residential buildings.
Nearby, masked men fired in the air as the first funeral processions of victims of Thursday's explosion drove slowly through the subdued streets of densely populated south Beirut.
As the country marked a day of official mourning, social media was flooded with pictures of the victims, and requests for information about people still missing.
Politicians from across Lebanon's diverse communities, including Sunni Muslims, Christians and Druzes, united to condemn the bloodshed in the Shi'ite neighbourhood, some visiting the area to offer condolences.
But in a sign of how the Syrian crisis has polarised Lebanon, there was celebratory gunfire in the mainly Sunni city of Tripoli on Thursday night and reports of people distributing sweets.
In an effort to limit sectarian tensions, Nasrallah called on Shi'ites to show restraint and said that takfiri groups were a threat to Sunnis and Shi'ites alike.
"These people kill Sunnis just as they kill the Shi'ites and they send suicide bombers to Sunni mosques just as they send them to Shi'ite mosques," he said, referring to al Qaeda-linked groups in Iraq, Pakistan and Somalia.
Speaking in an address to mark the seventh anniversary of the end of Hezbollah's 2006 war with Israel, Nasrallah also said he could not exclude that those radical Islamists were actually working for Israeli interests.
(Additional reporting by Laila Bassam in Beirut; Writing by Dominic Evans; Editing by Alison Williams)