"The intelligence service has the authority to conduct wiretaps for reasons of national safety, provided there is judicial approval," the source, who spoke on condition of anonymity, told AFP.
Greek authorities have been trying to prove a link between Golden Dawn and criminal activity following the murder last week of an anti-fascist musician, allegedly carried out by a self-confessed neo-Nazi.
"Both random and targeted wiretaps have been carried out for quite some time," the source added, noting that the operation preceded the musician's murder.
On Tuesday, at least 10,000 people demonstrated in Athens in an anti-fascist protest organised by unions and political parties.
Clashes with police broke out when demonstrators tried to approach Golden Dawn's central offices in Athens.
The rise of Golden Dawn, which won 18 parliamentary seats last year, has exposed Greece to international criticism a few months before it assumes the rotating European Union presidency in January.
Greek police have also been accused of routinely turning a blind eye to scores of migrant beatings, which the victims say were carried out by Golden Dawn supporters.
Several police officers have been suspended and an investigation has been ordered into claims that the neo-Nazi party enjoys wide support among the country's police force.
Golden Dawn, whose anti-corruption rhetoric had massively boosted its popularity in recent months, denies any involvement in violence, though several of its lawmakers have been involved in assaults.
The organisation says it has been unfairly targeted ahead of European parliament and municipal elections next year.
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