Ranariddh was travelling from the riverside town of Kampot to Preah Sihanouk as part of campaigning for controversial national elections set for July 29.
However, the secretary-general said "some of the prince's ribs were broken" and "one of his (right) leg's calf bones fractured" during the road accident. According to the local police chief, Prince Ranariddh had been rushed to a hospital in the capital Phnom Penh to treat his head wounds. Seven people were reported injured. Unlike his brother Sihamoni, who is seen as non-political, Ranariddh has had a turbulent political career.
A news agency close to the government reported Monday that Ranariddh was medically evacuated to Thailand overnight.
Nhep Bun Chin, a Funcinpec spokesman, said Ranariddh's condition had improved.
Health care in Cambodia has a poor reputation, and senior officials, including Prime Minister Hun Sen, as well as the well-to-do, often go overseas for serious medical problems.
Prince Ranariddh served jointly with Mr Hun Sen as Cambodia's prime minister from 1993 to 1997, when he was ousted in a coup led by the current prime minister.More news: Things that must happen for a Kawhi Leonard trade
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His royal background made him popular despite having a strained relationship with his father.
He was ousted in July 1997 and then fled overseas when long-simmering tensions between him and Hun Sen exploded into two days of bitter fighting in Phnom Penh between his forces and those loyal to Hun Sen.
Ranariddh was allowed to return to contest elections the following year but failed to repeat his success at the ballot.
Ranariddh's popularity and political fortunes declined afterwards and he was ejected from Funcinpec nearly a decade ago, only to return in 2015 after several attempted forays back into politics.
Ranariddh's Funcinpec was allocated most of the CNRP's parliamentary seats after its dissolution, and its leader, Kem Sokha, was arrested over accusations of plotting to take power with the help of the United States.
Ouk Phalla, a classical Cambodian dancer reported to be descended from a separate royal family branch, was Ranariddh's second wife.