Particle number concentration measurements of nanoparticles size ranging from 10 to 1090 nm was performed during the major firework event, Diwali, in the urban city Delhi. The fireworks during the festival emitted particles concentration ~9 × 104 cm−3 throughout the day. During the peak fireworks period, i.e., 18–20 hrs concentration was observed to be around 2 to 3 × 105 cm−3. During the pre- and post-Diwali period, the concentration of the particles was observed in the range of 6–7 × 104 cm−3. Accumulation mode particles contributed around 57% to the total concentration during Diwali day, and during peak hour of crackers bursting, the contribution reached to a maximum of about 60–70%. During normal period, contribution of accumulation mode to total particle concentration was around 31–40%. The particle size distribution showed peak concentration of particles around 150 nm on Diwali day, and on normal days, the peak found was around 100 nm. Post-Diwali period showed a 19% increase in total concentration of particles compared to the pre-Diwali period. NOx, NO2, and NO showed moderate correlation with accumulation mode (r2 = 0.4 to 0.6), and highest correlation (r2 = 0.7) was seen during the post-Diwali phase with large Aitken number concentration. Accumulation mode number concentration showed good correlation with CO (r2 = 0.6 to 0.7) during the pre-Diwali phase. The outcomes of the work will be helpful in distinguishing the different sources of nanoparticle emissions in the atmosphere. The findings of the study can be utilized to analyze the potential health effect due to inhalation of these particles and can help policymakers and authorities to formulate policies related to emissions during such firework events.