LUCKNOW, Nov. 24 -- A bomb threat letter discovered in a mall washroom on Monday afternoon triggered a massive security response across the city. The handwritten note warned that several Lucknow schools, government offices and key buildings would be blown up within 24 hours, prompting police to launch widespread checks with bomb and dog squads.

The alert comes at a time when security agencies across Uttar Pradesh were already in heightened vigilance mode following the recent blast near Delhi's Red Fort metro station on November 10. Officials said that the threat letter has led to further tightening of security measures across the state, including in the state capital.

After the letter was found amid regular weekday footfall at the mall,...