Norwegian Justice Minister Tor Mikkel Wara is to leave his post temporarily after his partner was arrested on suspicion of setting fire to his car.
Laila Anita Bertheussen, 54, was detained by Norway's PST security agency in the wake of five incidents at the minister's home.
She is currently suspected only in the 10 March arson, but will also be investigated over the other incidents.
Prime Minister Erna Solberg said it was a tragedy for Mr Wara and his family.
"This information came as a shock to me and to the entire government," Ms Solberg told a dramatic news conference in Norway.
The prime minister was told about the arrest about an hour before police made the details public.
PST head Benedicte Bjornland said Ms Bertheussen, whom she described as the minister's live-in partner, had been charged with carrying out an act that falsely indicated a crime had been committed.
She added that it was "too early to speculate on motives".
Ms Bertheussen could face a fine or a year in jail if found guilty.
Four months of incidents
The incidents allegedly targeting Mr Wara began 6 December when his house and car were daubed with the word "racist" in Norwegian.
Further alleged attacks took place in January and February, with another two this month.
The PST had faced criticism for failing to solve the series of crimes, which the prime minister said had made many other politicians and their families anxious.
Although he belongs to the populist Progressive Party, Mr Wara is seen as a moderate politician.
His party leader said everyone including Mr Wara was in shock.
'An attack on democracy'
Ms Bertheussen, who has been with Mr Wara for 24 years, had posted details of the incidents on her Facebook page, with critical remarks about police responses.
When the minister's car was set alight on Saturday night as it was parked outside their home in Oslo, Ms Bertheussen wrote on Facebook that she had been asleep at home on her own.
She has also been outspoken about a play entitled Ways of Seeing, which deals with racism. The production featured images of her home and the homes of other populist politicians, and she accused the Oslo theatre producing it of an invasion of privacy.
The prime minister's own handling of the incidents is being called into question. She had previously condemned the alleged arson attack as a threat against Norway's "open and democratic society".
And on Wednesday she implicitly criticised the theatre company, describing the play as "problematic for politicians' integrity".
When asked if she would apologise to the theatre in the wake of Thursday's arrest, the prime minster refused to respond, public broadcaster NRK reported.