Lebanon: Prominent Lebanese activist and Hezbollah critic Lokman Slim was found dead in his car with five gunshot wounds in south Lebanon Thursday, judicial and security officials said.

Slim, known for his opposition to Hezbollah, had not been heard from by his family since getting in his black rental car Wednesday evening to drive back to Beirut.

The 58-year-old was found dead in the driver's seat lying face down with a pool of blood in his car's passenger seat near the village of Addousiyeh.

Lead investigator Judge Raheef Ramadan ordered removal of his body to a government hospital in the city of Sidon where a forensic coroner reported he was hit with five bullets, four in the head and one in the back.

Preliminary information suggests he was killed between 2 am and 3 am, several hours after setting off in his car the previous evening.

A security official at the scene said Slim's ID, phone and gun were missing.

Prime Minister-designate Saad Hariri denounced the assassination, writing on Twitter: "We ... will continue the battle for freedom. Condemnation is not enough any more. Criminals must be revealed to stop the vengeful murderous machine."

Hezbollah's political opponents immediately pointed a finger at the group.

"Martyr Lokman Slim was assassinated in Hezbollah's area of influence, therefore it is the main suspect," former MP Fares Souaid said. "It should either tell the Lebanese what happened or bear direct responsibility for what happened."

Mustapha Alloush of Future Movement said Hezbollah's animosity toward Slim was well-known. "It seems it's the start of a liquidation campaign against (Hezbollah) opponents in the Shiite community."

SKeyes press freedom center said it feared a cover-up of the crime and more attempts to eliminate "symbols of free political thought."

Hezbollah, on its part, condemned the killing, adding in a statement, "We ask the judicial and security authorities concerned to work quickly to expose the culprits and punish them."

At Slim's family home in the southern suburbs of Beirut, where Hezbollah holds sway, family members sat in shock. Some wept in silence. A relative said they found out about his death from a news alert while at a police station to file a missing person report.

"What a big loss. And they lost a noble enemy too ... It's rare for someone to argue with them and live among them with respect," his sister Rasha told reporters, without naming Hezbollah.

She said he had not told her of any threats. "Killing is the only language they are fluent in," she added. "I don't know how we will go on with our work ... It will be hard."

President Michel Aoun called for a swift investigation to reveal the circumstance of the crime and those behind it. Caretaker Prime Minister Hassan Diab denounced "this heinous crime," which he said "shall not pass without accountability."

"It is a big tragedy," said Makram Rabah, a close friend and history lecturer. "Anyone who knows Lokman they know who his enemies are."

France's Foreign Ministry called the killing a "heinous crime" and demanded a transparent investigation. "France asks that the facts be clearly established and that all those who can contribute to establishing the truth contribute fully," Foreign Ministry spokeswoman Agnes von der Muhll said in a statement.

"It expects the Lebanese authorities and all Lebanese officials to allow the justice system to act efficiently, transparently and without interference."

US Ambassador Dorothy Shea strongly condemned the "horrific murder", saying it was "a cowardly attack on the principles of democracy, freedom of expression, and civic participation. It is also an attack on Lebanon itself."

"We also emphasize the need for an expeditious investigation of this and other recent unresolved killings so that the perpetrators of these acts are brought to justice," she added in a statement.

The United Nations envoy to Lebanon Jan Kubis wrote that he was saddened by Slim's murder, describing him as "an honest independent voice of courage."

He also said that, unlike the port blast, Slim's murder should be investigated in a "speedy and transparent way."

European Union Ambassador to Lebanon Ralph Tarraf tweeted: "Shocked & deeply saddened by the assassination of #LokmanSlim. My sincere condolences to his family and loved ones. We deplore the prevailing culture of impunity in #Lebanon in which such heinous acts take place and demand a proper investigation by the competent authorities."

French Ambassador Anne Grillo wrote in a tweet in Arabic that she received news of Slim's assassination "with deep sorrow and great concern."

Interior Minister Mohammad Fahmi, speaking to local TV station MTV, called it a "horrific crime."

Slim and his wife lived in the southern suburbs of Beirut, where they ran Umam Research and Productions, a research center and film production house. His family owns a publishing house and Slim hosted public debates and political forums and art shows.

Slim was often criticized by Hezbollah supporters for being instrumentalized by the United States.

The secular intellectual and pro-democracy activist had also made several documentary films with his wife Monika Borgmann, who had sounded the alarm on social media when her husband went missing.