Thawatchai Mekprasertsuk, commander of the Forensic Science Police Office, said the shots were fired from Mitr Maitree Road behind the stadium through all three entrances.
Some people fired on police from a tall building adjacent to stadium Gate 1 and, according to witnesses, a group of people carried sacks of stones into the dormitory of the Youth Centre.
They allegedly threw rocks at police from the building.
Metropolitan anti-riot police officers armed with rubber bullet rifles, took control of the situation but a group of men on the rooftop opened fire at police, injuring Pol Sgt Thanapol Nopluay who was shot at the right arm at an 80-degree angle, he said.
Pol Maj Gen Thawatchai said Wasu Suchantabutr, a protester, was shot dead near the Labour Ministry’s Social Security Office building, possibly from a ministry gate or a flat behind the stadium.
Police found 31 bullets and six bullet casings for 16 different models of rifles, he said.
Protesters tried to storm the Thai-Japanese stadium to disrupt the registration of party-list candidates for the February 2 general election.
Scores of people including police were killed and injured in the violence.