Siniora said he feared that mass protests would degenerate into violence. "This is a real concern," he said, adding the demonstrations would be "a jump into the unknown."
"I cannot really deny the fact that there might be some infiltrators" intending to stir trouble, he said of the demonstrations.
"We have to be very careful under the present circumstances in which tension is high ... people's behaviour might be interpreted differently and lead to unexpected results," he warned in an answer to a question.
Six ministers have resigned from Siniora's cabinet and the opposition, led by the Hezbollah, has demanded either a significant say in setting cabinet policy or the government's resignation and early elections.
The political crisis is also taking the form of a standoff between Washington, which backs Siniora and accuses Iran and Syria of seeking to topple the government, and Tehran, which backs Hezbollah.
Siniora extended a hand to Hezbollah and its allies in his speech to the diplomats, stressing that Lebanon should not become a battleground for regional and international powers.
"It is of paramount importance that we protect Lebanon's national interests, and not try to solve regional and international conflicts on our soil nor become an arena for the conflicts of others," he said.
Siniora acknowledged that the ministerial resignations had made the political situation "unhealthy", but he insisted that, with 18 of its 24 members, the cabinet still had constitutional authority.
He also reiterated that he would not resign.