Saad Hariri, who is serving as a caretaker until a new government is formed, spoke during a ceremony to mark the sixth anniversary of the assassination of his father, former Prime Minister Rafik Hariri.
Addressing thousands of his supporters in Beirut, Hariri accused Hezbollah and its allies of "lies, betrayal and lack of loyalty."
He called for mass protests on March 14 and made clear he would not take part in a unity government.
"Our mistake may have been that we extended our hand truthfully every time," Hariri said, speaking of his fallen government, which the U.S. and other Western powers supported. "But we were met every time with deceit, and our genuine intention was taken as a point of weakness and a sign of fear."
The ascendancy of Hezbollah is a setback to the United States, which has provided Lebanon with a total of $720 million in military aid since 2006 and has tried in vain to move the country firmly into a Western sphere and end the influence of Iran and Syria.
Hezbollah's opponents maintain having an Iranian proxy in control of Lebanon's government would be disastrous and lead to international isolation. The militant group has its own arsenal and is the country's most powerful military force.
Hariri was ousted as prime minister last month in a dispute over an investigation into who killed his father. An international tribunal set up to try his killers has yet to identify the perpetrators.
But the U.N.-backed tribunal is widely expected to accuse Hezbollah members of involvement in the killing. The militant group and its allies walked out of the previous government after Saad Hariri refused to denounce the tribunal.
Hezbollah may be in a position to block the court's work after its favored candidate, Najib Mikati, was named Lebanon's new prime minister. The group was able to secure enough votes of lawmakers for Mikati after a key politician and once a close ally of Hariri, Druse leader Walid Jumblatt, sided with the group, giving them the needed majority.
Mikati, who is struggling to form a new Cabinet, has called on Hariri and his allies to join in a unity government. He has said he will not end Lebanon's cooperation with the court - which Hezbollah has been calling for - unless there is national consensus.
But Hariri said Monday he would not join the government.
"We congratulate them on a majority that was hijacked by the intimidation of weapons and we congratulate them on a power that was stolen from the will of the voters," Hariri said.
He said he was joining the opposition and that commitment to the tribunal would be one of his priorities.
Hezbollah has denounced the tribunal as an "Israeli project" and a conspiracy against the group. Its leader, Sheik Hassan Nasrallah, has said he will "cut off the hand" that tries to arrest any of its members.
Hariri defended the tribunal and said he has full trust in the court.
"This tribunal ... will accuse members and will not do so randomly," he said. "It has to be based on evidence and proof."
U.N. Secretary-General Ban Ki-moon on Monday reaffirmed the U.N.'s commitment to the efforts of the tribunal, a day after President Barack Obama and Secretary of State Hillary Rodham Clinton urged the new Lebanese government to cooperate with the court.