Protesters marched through the main shopping streets
Police have criticised May Day anti-war protesters for pelting officers with missiles and causing criminal damage.
Sussex Police said three people were arrested, including one for assaulting an officer, amid some violent scuffles during the demonstration in Brighton.
Police say three officers suffered minor injuries like twists and sprains.
Campaign group Smash EDO, which wants local arms factory EDO MBM Technology closed, organised the protest which it described as a "huge success".
Smash EDO estimated about 1,000 protesters had gathered on Monday, but Sussex Police said it was about 500.
Brighton and Hove City Commander, Chief Supt Graham Bartlett, said: "Once again... we have seen unjustified acts of violence aimed at premises and police officers.
"Members of public including many visitors to the city have been clearly frightened and intimidated by a small minority of the group who have been verbally abusive, and throwing missiles at police.
Sussex Police said three officers were slightly hurt and three people arrested
"One member of the public was struck in the face by a missile and had to be treated by a police medical team."
Police also criticised organisers for not telling them how many protesters were expected and what route they would take.
Protests began with a street party in front of the Palace Pier before moving through the city centre and on to the factory to the north of the city.
A small number of demonstrators surged towards officers in riot gear and flares were let off.
And one small group scaled scaffolding in front of a branch of Barclays Bank and unfurled a banner.
Later groups of protesters gathered in the grounds of St Peter's Church and listened to music.
A number of others returned to the pier where they were surrounded by police in riot gear.
Smash EDO spokeswoman Chloe Marsh said the day was a "huge success".
"Large crowds arrived on foot and on bikes, bringing sound-systems, banners and a carnival dragon," she said.
At least 30 protesters had reported injuries and some were struck with batons by officers concealing their identity numbers, she said.
The march was a larger version of a demonstration regularly staged against EDO MBM Technology.
The weapons manufacturer has never commented on the protests.