Chavez said Tuesday his refusal to renew the license of Radio Caracas Television, which went off the air at midnight Sunday, is "a sovereign, legitimate decision."
He said another station Globovision — one of the few channels that is still harshly anti-government — had encouraged attempts on his life and could also face sanctions.
"I recommend (Globovision) take a tranquilizer, that they slow down, because if not, I'm going to slow them down," Chavez said in a speech.
Chavez did not elaborate, but said some broadcasters and newspapers are conspiring to spark unrest and warned that radio stations should not be inciting violence by "manipulating" public sentiment.
"A new destabilization plan is under way," Chavez said, calling for his supporters to be "on alert" and ordering officials to closely monitor media coverage.
Globovision's legal advisor Perla Jaimes told The Associated Press that Chavez had no legal basis to sanction the channel and said it would not be intimidated by the warnings.
"Globovision is not going to change its editorial line," Jaimes said. "We are only and exclusively doing our job ... we cannot stop transmitting the news. We cannot self-censor. We have to broadcast everything that is happening in the country."
Thousands — both Chavez supporters and opponents — staged separate marches in Caracas on Tuesday. The Chavez opponents chanted "freedom!" while government supporters said they were in the streets to reject an opposition attempt to stir up violence.
Government opponents remained in the streets into the evening as police stood by. The protesters earlier shouted insults against Venezuelan tycoon Gustavo Cisneros whose local TV channel, Venevision, has sharply toned down its coverage, and some accuse it of caving to government pressure.
Information Minister Willian Lara on Monday accused Globovision of encouraging an attempt on Chavez's life by broadcasting the chorus of a salsa tune — "Have faith, this doesn't end here" — along with footage of the 1981 assassination attempt against Pope John Paul II.
Globovision director Alberto Federico Ravell denied wrongdoing, calling the allegations "ridiculous." Globovision replayed footage of the assassination attempt during a retrospective of news events covered by RCTV during its 53 years on the air.
The government turned over RCTV's license to a new state-funded public channel, which showed a documentary on explorers in Antarctica, a children's program and exercise programs, interspersed with government ads repeating the slogan "Venezuela now belongs to everyone."
Chavez says it is a move to democratize the airwaves. He accused RCTV of helping incite a failed coup in 2002, violating broadcast laws and "poisoning" Venezuelans with programming that promoted capitalism.
International press freedom groups, the European Union, the Chilean Senate, Human Rights Watch and others have expressed concern about the move against RCTV. The U.S. State Department on Tuesday called on the Chavez government "to reverse policies that limit freedom of expression."
While Chavez made his speech Tuesday, thousands of students and opposition supporters marched to the offices of the Organization of American States, where they urged the body to take a stand chanting, "This is a dictatorship!"
Scattered protests were held in affluent parts of Caracas, as well as eastern Anzoategui state, central Carabobo state, western Zulia state and Margarita Island.
Thousands of government supporters held a rival march to the presidential palace accusing the opposition of trying to foment instability.
"RCTV was rubbish. Its programming was horrible, banal. Not even (the opposition) watched it," said Elena Pereira, an English professor at a state-funded university. "They want a reason to overthrow the government."
There were no reports of violence Tuesday, though on Monday police firing tear gas clashed with protesters in Caracas.
Justice Minister Pedro Carreno told reporters late Tuesday that 97 protests were registered nationwide in the last three days and that 182 people were detained, mostly university students and minors.
He said 19 police officers were injured but did not provide a figure for protesters.
Chavez did not mention CNN, accused by Lara on Monday of seeking to smear Venezuela. In a statement, CNN denied any campaign to discredit Venezuela and said it has "a long history of consistently balanced coverage" of the country.