Hassan Nasrallah said Saudi Arabia was holding Mr Hariri against his will.
He also accused the Saudis of inciting Israel against Lebanon.
The powerful Hezbollah Shia movement is an ally of Iran, which has been trading accusations with the Saudis of fuelling tension in Lebanon and the region.
Mr Hariri said in a TV broadcast from Riyadh on Saturday he was stepping down because of an unspecified threat to his life. He also attacked Hezbollah and Iran.
However, Lebanese President Michel Aoun and other senior politicians have demanded his return, amid suspicions that he is being held by the Saudis under house arrest and forced to do their bidding.
Mr Aoun has not accepted Mr Hariri’s resignation.
Mr Hariri has still not spoken publicly since his announcement.
In a televised speech on Friday, Mr Nasrallah said Saudi Arabia was trying to provoke fighting amongst the Lebanese.
“In short, it is clear that Saudi Arabia and Saudi officials have declared war on Lebanon and on Hezbollah in Lebanon, but I have to say this is a war on Lebanon,” he said.
Mr Nasrallah also accused Saudi Arabia of being prepared to pay “billions” to Israel for a military strike against Lebanon, describing this as the “most dangerous thing”.
At the weekend the Hezbollah leader had said that Mr Hariri had been forced to resign by the Saudis, but he repeated the allegations on Friday, saying that this was now “beyond any doubt”.
Saudi Arabia was attempting to remove Mr Hariri as prime minister and impose a new leadership on his political movement, he said.
BBC Middle East editor Sebastian Usher says Mr Nasrallah’s words were delivered as calmly as ever – but they will inevitably raise the temperature further as a growing number of countries outside the region try to calm the crisis. (Courtesy BBC)