Hizballah, which is backed by Syria and Iran, has branded the government a U.S. puppet.
"The protests will not stop until the government steps down," Hizballah spokesman Husayn Rahal told AP television earlier today in Beirut. "We want a government of national unity where all parties are represented. We think it is possible to achieve this goal. We will not accept our country being hostage to foreign [domination]."
Christian opposition leader Michel Aoun told the huge rally that Siniora and his cabinet must resign.
RFE/RL's Radio Farda correspondent Mahtab Farid reported from Beirut that Hizballah's leader has called for the protest to be nonviolent, but "a demonstration, a peaceful demonstration, just to show their opposition to the government."
Lebanese security forces, backed by armored cars, cordoned off government buildings in the area. Organizers have vowed to follow up today's rally with an indefinite sit-in near government offices.
The pro-Syrian Hizballah has threatened for weeks to send its supporters into Beirut's streets to bring down the government. But it delayed doing so after the assassination on November 21 of Lebanese Industry Minister Pierre Gemayel.
That is because the assassination of Gemayel, a vocal opponent of Syrian influence in Lebanon, has greatly increased tensions. Gemayel's supporters accuse Damascus of having a hand in the killing, a charge Syria denies.
Struggle Over Syrian Influence
At the heart of the political crisis is Lebanon's continuing domestic struggle over Syria's influence in the country.
Damascus withdrew its troops from Lebanon in 2005 following massive anti-Syrian protests over the assassination the same year of former Prime Minister Rafiq Hariri, a leader of the anti-Syrian camp.
But Western officials say Syria remains deeply involved in Lebanon's internal affairs -- largely through Hizballah, which Damascus and Tehran help fund and arm.
One of the sparks for the current crisis is a dispute over whether Lebanon should create a tribunal to look into the 2005 Hariri assassination -- an assassination which UN investigators say Syrian officials helped plan.
Hizballah and another pro-Syrian party withdrew their members from Siniora's cabinet in protest over the tribunal proposal last month.
But the resignations of the six cabinet members have not been enough to dissuade the government, which says it will submit its tribunal plans to the parliament soon for approval. The proposal won the endorsement of the UN Security Council last week.
But Hizballah is now raising the stakes by moving the battle to the streets. If another two members of the cabinet can be persuaded to resign, the government will fall.
However, anti-Syrian groups are also mobilizing their supporters. Siniora said in a nationwide television address on November 30 that the government has the people's support.
"This is a democratic country and they are free to demonstrate any way they want," Siniora said. "This is what democracy is all about. In regards of bringing down the government, they wouldn't be able to bring down the government because the government has the support of the majority of the parliament and the majority of the Lebanese people."
Radio Farda correspondent Farid says an anti-Syrian coalition, the March 14 Group, is planning a counterrally today around 3 p.m. local time. "This is the pro-Sunni, pro-Christian group; they are the anti-Hizballah group," Farid noted.
Lebanese society is fragmented politically, with lines cutting not only between religious communities but also across them.
That creates ever-shifting and often unlikely alliances, including one currently between Hizballah and a Christian party led by former Prime Minister Michel Aoun.
The political fragmentation makes the outcome of almost any political crisis in Lebanon extremely difficult to predict.
(with agency reports)
Former Lebanese Prime Minister Rafiq Hariri
Lebanese demonstrating in Beirut on the first anniversary of Hariri's slaying in February (epa)
POINTING AT SYRIA. The February 2005 assassination of former Lebanese Prime Minister Rafiq Hariri shook the fragile political situation in Lebanon and threatened the entire region. The international community have been increasingly vocal in accusing Syria of involvement in the killing and in demanding that Damascus cooperate with a UN investigation....(more)