Waving placards reading "Nuclear go zero," and "Abolish nuclear, save Taiwan," protesters rallied outside the presidential office in Taipei as Japan marked the seventh anniversary of the Fukushima disaster.
Protesters were worried by a recent decision by the high-level Atomic Energy Council to allow state-owned Taipower to restart a reactor at a facility near Taipei, pending parliament's final approval.
The reactor has been offline since May 2016 after a glitch was found in its electrical system, which the company claims is fixed.
And anti-nuclear groups question if President Tsai Ing-wen's Democratic Progressive Party will keep a promise to phase out nuclear energy.
"It would be violating the spirit of creating a nuclear-free homeland by 2025," said Tsui Shu-hsin of rally organizer Nuclear Go Zero Action Platform of the prospect of restarting the reactor.
Lawmaker Huang Kuo-chang, head of the opposition New Power Party, echoed that sentiment. "The government should move forward, not backward - and restarting the reactor would be a regression," he said at the rally.
Taiwan now generates about a fifth of its energy from three nuclear plants. Although concerns have grown over power sufficiency after island-wide failures in August, many people remain against nuclear energy.
In 2014, authorities were forced to seal off a nearly-completed fourth nuclear plant amid opposition.
Some at yesterday's rally wore sunflowers on their hats to symbolize clean energy.
"I would rather the government restrict the use of electricity than relying on nuclear energy, which is unsafe and generates a lot of waste," said college student Yao Hsing-yu, 22.
Taiwan started annual anti-nuclear rallies to commemorate Japan's nuclear disaster on March 11, 2011, when the Fukushima energy plant was hit by a tsunami following an earthquake and went into meltdown.
Taiwan, like Japan, is prone to quakes as the island is on fault lines. Seventeen people were killed and 300 hurt last month when a 6.4-magnitude quake hit Hualien.
"Nuclear facilities are unsafe as Taiwan has many quakes," said Fan De-lu, 40. "The government needs to take the lead to develop alternative and green energy."