The pro-government Fresh News website quoted tax authorities as saying that the tax bill stands even if the paper shuts down and that whoever is responsible at the publication would be barred from leaving Cambodia until the money is paid.
"Kem Sokha is in police custody at city hall".
Interior Ministry spokesman Gen. Khieu Sopheak said Kem Sokha was being held at the Tropeang Phlong prison facility in Tbuong Khmum province, some 130 kilometres (80 miles) east of Phnom Penh.
The government appeared to have based part of its claims against Kem Sokha on a video clip that shows the opposition leader giving a public speech in which he describes a grassroots political strategy to challenge Hun Sen with US support.
The arrest "sends a very clear signal that democracy in Cambodia is under severe threat", she said. Kem Sokha allegedly spoke on CBN about how the U.S.is supporting him to overthrow the government.
Sokha is facing charges of treason and espionage under article 443 of the country's Criminal Code, a government statement said.
Kem Sokha was elected as CNRP President in March 2017 to replace Sam Rainsy who stepped down in the previous month. The CNRP could therefore be dissolved if Sokha is found guilty.
The Cambodian government has previously threatened to shutter media outlets they say jeopardise "stability" in the country.
"We have been a burr in Hun Sen's side for the entire time that we have been operating", said The Cambodia Daily's American editor-in-chief, Jodie DeJonge.
Monovithya, who is also a member of the Cambodia National Rescue Party, says police did not have a warrant for her father's arrest.
But the strongman has no plans to relinquish that power anytime soon, and when the opposition party did better than expected during June's local elections, Hun Sen started cracking down on dissent and free speech.
The arrest of Kem Sokha comes after the government cracked down on NGOs and critical media.
An independent newspaper, The Cambodia Daily, has been ordered to halt publishing by Monday because of a tax dispute, and at least 15 radio stations have been ordered to stop broadcasting programming from the Voice of America and Radio Free Asia.
The newspaper cried foul in its closure statement on September 3: "In an ordinary process, matters in dispute would be resolved after an audit and private negotiations".
"Instead, the Daily has been targeted with an astronomical tax assessment, leaks and false statements".
The paper, which is often critical of the government, said it had been destroyed by the $6.3m (£4.9m) bill.
On August 23, the government closed the National Democratic Institute (NDI), a US -funded NGO, and ordered its expatriate staff out of the country.
The arrest appeared to be part of a broader push by the government of Prime Minister Hun Sen, one of the world's longest-serving rulers, to eliminate his opponents and silence critics ahead of the 2018 vote. A former Khmer Rouge cadre, he has become one of China's closest regional allies and has been making increasingly strident criticisms of the US.