Leaders in the pro-government March 14 bloc had urged hundreds of thousands to take part in the al-Hariri commemoration to show their rejection of alleged Syrian efforts to regain influence in Lebanon.
Hezbollah, which is supported by Syria and Iran, has led an opposition political bloc against March 14 for the last three years.
Many of those taking part in Thursday's rally carried Lebanese flags and photographs of al-Hariri as well as other politicians and political figures killed in attacks blamed on Syria.
Al-Hariri was killed in a car bombing on Beirut's seafront in 2005, sparking mass protests and the withdrawal of Syrian troops after 29 years.
Syria denies any links to a series of killings across Lebanon in the past three years.
"In 2005 you took to the streets to force them out. In 2008 you must do the same so that they don't come back," read messages on billboards around Beirut.
In a speech to the rally, Saad al-Hariri, the former prime minister's son and the parliamentary leader of the ruling bloc, said that three years ago forces tried to "assassinate Lebanon".
"Here you are today, gathering once again, responding to the call of freedom, sovereignty and independence," he told the crowd.
Schools and universities were ordered to shut on Thursday, while most businesses were set to close as the government declared a holiday.
Concerns that the two events will spark unrest has prompted foreign embassies to urge their citizens to observe extreme caution and avoid travel in Beirut.
Staying at home
Al Jazeera's Rula Amin in Beirut said that many Lebanese would welcome the fact that it was raining, hoping that it would keep the turnout at the rallies low and reduce the chances of violence.
"Many people are staying at home, worried and just hoping that today passes without any problems," she said.
"The point that March 14 wants to make here is that they still have support and are willing to fight if necessary."
The day of mourning for both the March 14 ruling bloc and its opponent Hezbollah comes amid continuing differences in Lebanese politics.
There has been a power vacuum in Lebanon since November, with opposing political factions failing to agree on a new president and the distribution of key cabinet portfolios.
Al-Hariri called for the parliamentary vote to confirm General Michel Suleiman as president, which has been postponed 14 times, to go ahead.
"We want a president for the republic," he told the rally.
Hezbollah has accused Israel of killing Moghaniyah, which Syria, a supporter of Hezbollah, branded a "cowardly and terrorist act".
Hassan Nasrallah, the Hezbollah secretary-general who is in hiding after the 2006 war with Israel, addressed mourners through a video broadcast at Moghaniyah's funeral on Thursday.
He blamed Israel directly for Moghniyeh's death, and said that the July 2006 war with Israel was continuing, although not on a military level.
"The July war is still continuing - until now, no ceasefire has been proclaimed. This war is continuing at the politcal level, at the media level, at the financial and material level, at the security level," he said.
"It is supported by the same countries that supported Israel in the July war. This is why [Moghaniyah] was assassinated."
Nasrallah said that the death of Moghaniyah would only strengthen the resistance against Israel.
He said that the death of Moghaniyah on non-Lebanese soil meant that Hezbollah was now ready to fight Israel in areas other than Lebanon.
Israel has ordered its military, embassies and Jewish institutions around the world to increase security measures in case of revenge attacks, Israeli officials said on Thursday, on condition of anonymity.
The Israeli prime minister's office denied allegations that it had played any role in Moghaniyah's killing.
"Israel rejects the attempts of terror elements to attribute to Israel any involvement in this incident," Ehud Olmert's office said in a statement on Wednesday.
Iran also accused Israel of carrying out the attack.
|Moghaniyah is accused of involvement in the |
bombing of a US embassy in 1983 [Reuters]
However, the US welcomed Moghaniyah's killing, saying that he had been responsible for many deaths.
"The world is a better place without this man in it. He was a cold-blooded killer, a mass murderer and a terrorist responsible for countless innocent lives lost," Sean McCormack, the US state department spokesman, said on Wednesday.
Also known as Hajj Radhwan, Moghaniyah was widely suspected of being behind a wave of Western hostage-taking in Lebanon in the 1980s, claims denied by Hezbollah.
According to the US and the West, he was a "top terrorist" and was involved in the bombing of the US embassy in Beirut in April 1983.
The US also claims he was involved in the killing of hundreds of US marines and French paratroopers in simultaneous truck bomb attacks in October 1983.