BEIRUT - 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 fueled 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-Qaida, many of whom are fighting with Syrian rebels against President Bashar Assad."If you think by killing our women and children ... and destroying our neighborhoods, 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.Defense 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 neighbor'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 bombingThursday'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.Politicians from across Lebanon's diverse communities, including Sunni Muslims, Christians and Druzes, united to condemn the bloodshed in the Shi'ite neighborhood, some visiting the area to offer condolences.But in a sign of how the Syrian crisis has polarized 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-Qaida-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.Commenting on the incident last week in which four IDF soldiers were injured on the Lebanese border, Nasrallah, whose organization claimed credit for the explosions, vowed that any Israeli soldier who will try to enter Lebanese territory will lose his legs."The era of Israeli tourism on the Lebanese border has ended forever, we are stronger than ever," he said, according to Army Radio.