The assailant entered the Golden Temple in northwestern Amritsar city as a visitor and attempted to shoot Sukhbir Singh Badal, the president of a Sikh political party.
Badal's security tackled the gunman after he took his weapon from his waist and the sole shot he fired off missed its target, instead hitting a marble pillar.
"Security of Sukhbir Singh Badal has been tightened. The assailant is in police custody and investigation is in process," senior Punjab state police officer Harpal Singh told reporters.
Badal, 62, was at the temple to serve a punishment imposed by the faith's hierarchy for alleged "mistakes" committed while his party was governing Punjab state in the decade up to 2017.
He had been ordered to sit at the Golden Temple's entrance holding a spear since the beginning of the week as an act of contrition.
The Golden Temple -- a gleaming structure in a large artificial pond, revered by Sikhs the world over -- has been the scene of violence in the past.
Indian special forces stormed it in 1984 to remove Sikh militants that had barricaded themselves inside during an insurgency demanding an independent Sikh homeland carved out of India.
Hundreds were killed, many of them civilians, when the army stormed the temple, and outraged Sikhs accused soldiers of religious desecration.
In the aftermath later that year, then prime minister Indira Gandhi was assassinated by two Sikh bodyguards, leading to brutal reprisals that killed thousands of Sikhs around India.
