Israel plans to maintain a high level of alert worldwide in the coming weeks as Hizbullah's No. 2, Naim Qassem, threatened Saturday to avenge last month's assassination of the Shi'ite group's chief operations officer Imad Mughniyeh in Damascus. The traditional 40 days of mourning for Mughniyeh ended on Saturday and Israel raised security worldwide - in Israeli missions and Jewish institutions - out of concern that Hizbullah will try to avenge the February 12 car-bombing by striking at a Jewish or Israeli target abroad. On Friday, the IDF beefed up its presence on the Lebanese border, diverting units from training facilities and other operations. While Israel has denied involvement in the terror-chieftain's death, Qassem renewed charges on Saturday that Hizbullah had "100 percent solid evidence that Israel killed Mughniyeh." The Lebanese press published a Hizbullah statement about a rally scheduled for Monday in Beirut to commemorate Mughniyeh. In conjunction with the high level of security in Israel - police beefed up their presence at Purim festivities over the weekend - the Shin Bet (Israel Security Agency) has decided to dramatically increase its level of security surrounding Israeli embassies, consulates and other offices worldwide. The Shin Bet has also reinforced security and altered arrangements for El Al Israel Airlines flights around the world. One of the main concerns is that terrorists will try to down an Israeli passenger jet with a shoulder-to-air missile just as al-Qaida tried to do in Kenya in 2002. Defense officials announced over the weekend that Israel would begin outfitting its civil airliners with a defense system designed to thwart missile attacks. The officials said the system fired flares that distracted an incoming missile's heat-seeking mechanism. It will be installed first on planes flying to destinations considered dangerous, especially those in Africa and parts of Asia. Test installations were conducted several years ago, but the widespread fitting of the country's commercial fleet was held up until this month by an argument over who would foot the bill - the government or El Al and Arkia airlines. But Hizbullah threats have now prompted the government to agree to pay most of the cost.