The victims all worked at the plant 100km (62 miles) east of Porto.
The factory's owner was among the dead and several members of his family were either dead or missing, said local mayor Francisco Lopes.
Some of the victims were found outside the factory, apparently propelled from the building by the force of the blast.
An initial explosion happened shortly after 17:30 (16:30 GMT) on Tuesday, followed by two more blasts, reports said.
Five bodies were found late on Tuesday and emergency workers found a sixth victim some 200m (650ft) from the site, minister Jorge Gomes told Portuguese media. The search was being extended to a radius of 800m, he added, because of the scale of the explosion.
The factory was described as a family business.
As investigators tried to identify the victims, the mayor said that almost all of the eight caught up in the blast were members of Egas Sequeira's family. Mr Sequeira was among those feared dead along with one of his daughters and a son-in-law, Portuguese media said.
The owner's wife had left the building just before the explosion and survived but two other family members were among the missing.
Officials were initially unable to enter the site because of the potential risk to their safety. Portuguese President Marcelo Rebelo de Sousa was due to visit the scene on Wednesday.

Both sides had chances before the Pars' Ryan Wallace drilled a low shot into the bottom corner with 15 minutes left.
The lead last just two minutes as Ross Davidson got the last touch on a free-kick into the area.
The home side dominated the closing stages but could could not deny Rovers, who remain in fifth place.
Rovers remain level with Airdrieonians, who drew at home with Forfar Athletic.

Daly spent two seasons at Ibrox after leaving Tannadice, where he has been training since pre-season.
"I would never say never," the 32-year-old said of the possibility of signing for the Scottish Premiership club.
"If they came to me and were interested, I would definitely consider it."
Daly spent six years at Tannadice and ended as club captain.
"I am currently training at Dundee United," he told BBC Scotland.
"When pre-season came about and I still hadn't got a club, I rang manager Jackie McNamara and asked if I could go in.
"They are a good club with some fantastic young players.
"I would never say never, but United would need to want me first."
The United States appears to be a possible destination for Daly.
"I am probably looking further afield than Scotland," he said. "I'm looking abroad.
"At the moment, there are a few clubs interested, but they have their full forum of allocated players so they need to maybe lose one or two of them before they can move.
"It is looking like January before the foreign market can come about, so I might need to get something short term until then."
Daly had no regrets about his move to Rangers, who helped the Ibrox side win Scotland's third tier before dropping to the bench for much of last season in the Championship.
"I was out of contract with Dundee United and at the time they were only prepared to offer a one-year contract," he said.
"When I spoke to Ally McCoist, they were prepared to offer two years.
"A lot of people say you are dropping down the divisions, which is a fair point, but it is going to a club of Rangers' stature, history, fan base, playing at Ibrox every second week and training at Murray Park every day.
"I was just delighted to go to a club like that and play for a man that I really respected in Ally McCoist."
Six people died and another two were missing after an explosion on Tuesday evening destroyed a fireworks factory near the Portuguese town of Lamego.
[NEXT_CONCEPT]
Scottish League One leaders Dunfermline Athletic were held at home by Albion Rovers but still moved 11 points clear at the top of the table.
[NEXT_CONCEPT]
Jon Daly has not ruled out a return to Dundee United, but the striker thinks his future probably lies abroad following his release by Rangers.