President Xi Jinping and state leaders sheltered under umbrellas in driving rain at a morning ceremony in the expansive square, while school children and a military band were wrapped in plastic ponchos under a leaden sky.
But as the weather cleared and the square reopened to the public, tourists crammed in, many holding flags and wearing Mao caps.
The scene was dominated by an incongruous giant 13-metre-high (43-feet) vase of fake flowers and fruit, an installation which has attracted criticism online as an unnecessary expense.
Transport networks were jammed full of holidaymakers as the 10-day travel period, which started Saturday, hit its peak.
Vacationers were expected to log a total of 430 million trips, the official China News Service said, with tourist sites and highways across China expected to see large crowds over the coming week.
Railway ticket halls and platforms were filled with travellers, weighed down with infants, luggage and cameras.
Around 400,000 passengers left Beijing from its four train stations on Monday, the Beijing News reported, citing city railway authorities.
A train station in the eastern city of Nanjing was expected to send off 610,000 passengers in the first four days of the holiday, with ticket halls and platforms at stations around the country filled with travellers.
Passengers at a station in northern Taiyuan city stood backpack-to-backpack as they shuffled toward their platforms, streaming around rows of metal seats and filling the aisles of the vast hall, images from China News Service showed.