TOKYO — For the first time since antinuclear rallies began months ago outside Prime Minister Yoshihiko Noda’s office, a dozen protesters were allowed inside on Wednesday for a half-hour meeting that the fledgling movement hailed as a victory. The meeting comes at a time of growing antinuclear sentiment in Japan, and with elections expected this year.

Mr. Noda, who angered demonstrators by dismissing their weekly rallies as “loud noise,” had been under public pressure to meet with them face to face.

The demonstrators are calling for a shutdown of the reactors at the Oi nuclear power plant in western Japan — the first reactors to be restarted since the nuclear accident at Fukushima in March 2011 — and for Japan to decommission its 54 other reactors. They say the reactors are not safe to restart, given the country’s frequent earthquakes and the government’s failure to prevent the Fukushima disaster.

The rallies outside Mr. Noda’s office in the heart of Tokyo have grown from crowds of several hundred to tens of thousands since the prime minister gave the go-ahead for restarting the Oi reactor in July. “Saikado hantai!” — “Oppose the restarts!” — is now the rallying cry at the protests, which the police say have swelled to include almost 100,000 people, although organizers say the turnout is almost twice that.