The protests on Monday come as the government warned more attacks were likely and acknowledged it was unable to stop them.
May Chidiac, a longtime news anchor and a political talkshow host, lost her left arm and leg from a bomb that exploded in her car in the town of Jounieh, north of Beirut, on Sunday.
Doctors said Chidiac remained in stable condition in intensive care.
Chidiac, in her 40s and a famous household face, works for Lebanese Broadcasting Corporation, a popular private station at home and abroad that is known to oppose Syria's role in Lebanon.
On Monday afternoon, politicians and journalists, including LBC staff, joined an estimated several thousand students around the martyrs' statue on a seaside square in central Beirut in observing a moment of silence and a silent prayer for Chidiac's recovery.
Flanking the bullet-scarred statue, a symbol of Lebanon's quest for independence, many waved Lebanese flags, and those of Christian groups such as the Phalange Party and the Lebanese Forces as well as the logo of LBC, which is a Christian station.
Broadcaster Chidiac had her left
arm and leg amputated
An estimated 50 veiled Shia Muslim female students from Hizb Allah carried pictures of Chidiac.
A man carrying a sign reading "Enough" along with a symbol of the cross and the Quran.
The attack "aims to sow sectarian sedition and drive a wedge in national unity", Daniel Spiro, a student leader from the Lebanese Forces, told the crowd.
"We will confront the terrorism with the weapons of national unity and we will confront the criminals with a peaceful, political revolution."
Pens in the air
Earlier on Monday, students protested at the private Notre Dame University, where Chidiac teaches journalism, and other university campuses.
Some lifted pens in the air or waved flags; others sang the national anthem.
"We are facing some kind of phantom, a certain person or party, who are professionals and who have hatched a terrorist plot and are carrying it out"
Lebanese Interior Minister
Student groups representing Christian and Muslim political factions opposed to Syria organised Monday's protests to demand action.
"Let us immediately act to confront this terrorist monster and finish off the remnants of the security system," the students declared in a statement, referring to security members Syria and its Lebanese allies have long used to control Lebanon.
As politicians, ambassadors and dignitaries streamed to the Beirut hospital where Chidiac is being treated, LBC chairman Pierre Daher said the station will not be cowed by the attack and criticised the government's failure to halt the wave of bombings.
Admission of helplessness
Interior Minister Hassan Sabei warned of an ongoing "terrorist plot" to destabilise the country and acknowledged the government was finding it difficult to break the chain of attacks.
The government's admitted helplessness in tracking down the perpetrators of the bombings lends credence to public fears of increased violence as the country anxiously awaits the findings of a UN investigation into the murder of former prime minister Rafiq al-Hariri, a probe which could possibly implicate Syria and some of its allies in Lebanon.
Lebanese security forces are
struggling to stop the violence
Sabei said security agencies would intensify their intelligence gathering efforts.
"We are facing some kind of phantom, a certain person or party, who are professionals and who have hatched a terrorist plot and are carrying it out," he said after a meeting with security agencies led by Prime Minister Fuad Siniora.
"As long as we have not apprehended any person, the plot will continue," Sabei said.
Lebanon meanwhile will ask the United States and France to help train its security forces to stop the country from sliding into chaos.
A close aide to Prime Minister Fuad Siniora said the government would ask the Americans and French to help
instruct its security services and provide logistical support.
"The issue has been on the government's agenda and a list of needs has been made, but it is more urgent now after Sunday's attack," he said, speaking on condition of anonymity.
The attack "aims to sow sectarian sedition and drive a wedge in national unity"
Lebanese Forces student leader
Siniora said on Sunday he had asked the US embassy for assistance in Chidiac's case.
He said a team of US investigators was due in Lebanon on Tuesday to help out with the investigations, assistance the US has given before.
Opponents of Syria's role in Lebanon have accused remnants of the Lebanese-Syrian intelligence apparatus of being behind the al-Hariri assassination and subsequent bombings.
UN Secretary-General Kofi Annan called the latest attack a "callous act" and European Union foreign policy chief Javier Solana condemned it.
But Syrian officials, the target of the UN investigation into al-Hariri's assassination that could shake President Bashar al-Assad's government, had no comment on the latest attack to hit Lebanon despite accusations of Damascus' involvement.
Three state-run newspapers and the official news agency reported the bombing prominently, while Syria's state-run Journalists' Union condemned it.