The thieves used a rope to climb down into the 15-feet deep well. They entered it by breaking the grill-mesh of its window, Amritsar Police Commissioner S S Srivastava said.
"One person has been detained. It is suspected that two petty thieves were involved in the incident. They stole about Rs 3,000, mainly in coins, from inside the well," he said.
Srivastava said the site has no CCTV camera surveillance and a watchman had been hired by authorities for the site's security.
The watchman told police that he did not notice any movement last night and spotted the rope used by the thieves today morning, the officer said.
The Martyr's Well has been dry for years, he said.