BEIRUT, Lebanon (CNN) -- The head of Hezbollah threatened "open war" against Israel as mourners gathered in Beirut Thursday for the funeral of a senior commander in the Islamic militant group organization killed this week.
In a separate rally in the Lebanese capital, supporters of the former Lebanese prime minister Rafik Hariri gathered to mark the third anniversary of his assassination in a massive car bomb attack.
Officials estimated hundreds of thousands of people were taking part in the two rallies with around 10,000 police and security force members working through pouring rain to keep the peace between the rival crowds.
Addressing supporters in a pre-recorded message delivered via a giant video screen, Hezbollah leader Sheikh Hassan Nasrallah accused Israel of being behind a blast in the Syrian capital, Damascus, which killed militant commander Imad Mughniyeh on Wednesday.
Israel has flatly denied involvement in Mughniyeh's death
"You have crossed the borders," Nasrallah said, accusing Israel of extending the simmering conflict between Israel and Hezbollah beyond the Lebanese frontier.
"With this murder, its timing, location and method -- Zionists, if you want this kind of open war, let the whole world listen: Let this war be open."
Israel's military chief Thursday ordered his forces to "take the necessary steps" to protect the country's border with Lebanon. Israeli Chief of Staff Maj. Gen Gabi Ashkenazi ordered the military to be prepared in the air, sea and land in order to protect the northern border and other Israeli interests.
Hezbollah militants are active in Lebanon, and have pounded northern Israel with Katyusha rockets in the past, sparking a war with Israel in the summer of 2006.
In Washington, State Department spokesman Sean McCormack called Nasrallah's statements "alarming."
"Hezbollah has a long record of carrying out violent acts, acts of terrorism around the globe," he said. "You just look at the history of Mughniyeh and his group. You have a pathway of violence that stretches from Buenos Aires to Kuwait and a lot of places in between."
Mughniyeh's coffin, draped in a yellow Hezbollah flag, was carried through the streets amid thousands of Hezbollah supporters and was due to be buried later in the day.
Mughniyeh, whom intelligence sources described as one of the craftiest and deadliest terrorists in the world, managed to elude capture for decades by changing his appearance and covering his footsteps.
He was killed by a blast in a residential section of the Syrian capital, Damascus, Hezbollah's television station, Al-Manar, announced on Wednesday.
Meanwhile, Hariri's supporters began gathering early Thursday in Martyr Square despite rainy weather. Wearing raincoats and holding umbrellas, they carried Lebanese flags and pictures of the former prime minister.
Schools and universities were ordered to close as the government declared Thursday a holiday to commemorate his death.
The emergence of the two large and violently opposed political factions onto the streets have raised fears of street violence.
But CNN Beirut Bureau Chief Brent Sadler said the two groups were keeping to their separate areas for now. But violence could come as demonstrators go home in the evening, some to neighborhoods where members of both groups live, he said.
Hariri supporters blamed his death on Syria. The outcry over the former prime minister's assassination eventually drove Syrian forces out of Lebanon and prompts mass rallies on the anniversary each year.
Organizers claimed Thursday's rally rivals the so-called Cedar Revolution, when more than one million people took to the streets after the assassination.
U.N. investigators concluded in 2006 that Hariri's death may be linked to high-ranking Syrian officials, who back the Shiite militia Hezbollah. Syria has denied any involvement in the killings and said the U.N. tribunal investigating Hariri's death is a violation of its sovereignty.
In the past two years, four members of parliament have been assassinated. Since December, bombings there have also killed Lebanon's top anti-terror investigating officer and the head of operations for the Lebanese army.
Attacks have escalated while Lebanon is in the midst of a political crisis. Pro- and anti-Syrian lawmakers in parliament are locked in a battle to elect a president.
The nation has been without a president since Nov. 23, when the pro-Syrian Emile Lahoud stepped down at the end of his term.
In Washington Wednesday, several top U.S. officials expressed relief at the news of Mughniyeh's death. Western intelligence agencies have long suspected he took part in the 1983 bombing of the U.S. Embassy in Beirut, which killed 63 people, as well as the truck bombing that year of the U.S. Marine barracks in the city. That attack killed 241 people and led to the U.S. military withdrawal from Lebanon.
Before the terrorists attacks of Sept. 11, 2001, U.S. officials blamed Mughniyeh for the deaths of more Americans around the world than any other terror suspect. Just a month after that attack, the FBI debuted its "Most Wanted Terrorists" list and placed Mughniyeh on it. E-mail to a friend