LG released 100 helium balloons, each with a free smartphone voucher, at the so-called G in the Cloud event, which took place in an outdoor park in the South Korean capital city.
The phones, which sell for KRW 950,000 in South Korea ($851; £550), would be given to people in possession of the voucher, the company said.
Customers arrived with BB guns to shoot down the balloons and surged forward when they were released.
LG has taken responsibility for the situation and said it would cover related medical costs.
“LG Electronics deeply regrets that a number of Korean participants were injured during an outdoor promotional event in Seoul, Korea,” a statement from the firm said.
“We can confirm that seven participants were hospitalised and, although none of the injuries were serious, LG takes full responsibility for the unfortunate situation and has offered to cover all related medical expenses.
“LG is investigating the incident to ensure that such an occurrence can be avoided in the future.”
The company added that other planned events had been called off because of safety concerns.