BEIRUT (Reuters) - The Saudi-owned pan-Arab television channel al-Arabiya al-Hadath said on Saturday an assassination plot was foiled against Lebanese leader Saad al-Hariri in Beirut days ago, citing an unnamed source.  Lebanon s Internal Security Forces (ISF), responding to reports that one of its branches had foiled an assassination attempt on Hariri, denied that it was the source of the reports and said it  had no information on this . Hariri traveled to Saudi Arabia on Friday and announced his resignation as prime minister on Saturday in a speech condemning Riyadh s regional foes Iran and Hezbollah and saying he feared assassination.   Those who planned to assassinate prime minister Hariri deactivated the observation towers while his motorcade was passing by,  Arabiya cited the source as saying. 