The people left standing outside in December were African-American, Hispanic, and Cape Verdean, said the suit, announced yesterday. The birthday celebrant, who was white, was already inside. She came out and tried to intervene, but her efforts were in vain. Owner Caron O’Neil, the suit said, refused to let the other guests come inside.

“When it became clear that they were not going to be allowed into Peggy O’Neil’s to celebrate their friend’s birthday, the friends left the bar feeling hurt, confused, and embarrassed,’’ according to the suit, filed in Suffolk Superior Court.

Attorney General Martha Coakley’s office alleges that the Dorchester establishment engaged in a discriminatory and unlawful pattern of not admitting minorities. The diverse birthday party arrived in two groups that were turned away separately. A third group of black women was allegedly denied entry in April.

The suit seeks monetary damages, civil penalties, antidiscrimination training, and the creation of an antidiscrimination policy.

“No one who lives, works, or visits Massachusetts should be subjected to discrimination,’’ Coakley said in a statement.

The O’Neil family denied the allegations, insisting that bias and inequity are not tolerated on the bar’s premises.

“Absolutely, we do not discriminate against anybody,’’ said O’Neil, named in the suit as the bar’s owner. “We’re a Dorchester neighborhood bar. That’s our customer base.’’

Peggy O’Neil’s has been in the red brick and white building for more than 50 years. The pub, which sits on a stretch of Dorchester Avenue bustling with Vietnamese stores and restaurants, is named after the family matriarch, Margaret “Peggy’’ O’Neil, who died in May.

Tracy O’Neil, Caron’s sister, said a diverse group of men and women walk through the doors and sidle up to the bar seven days a week.

“It’s just a diverse crowd; it’s like anywhere,’’ she said yesterday as a small group of regulars had afternoon cocktails and another patron played Lotto.

The suit alleges that Amilton Baptista and an African-American friend did not receive a warm welcome when they arrived Dec. 17.

They waited in line and watched as the bar’s staff let in up to 10 white customers. But Baptista and his friend were denied entry when their turn came to walk through the doors, the lawsuit said.

Both men, the lawsuit said, showed the bouncer their identification cards, proving they were well over the legal age to enter the bar.

They asked how much it cost to get in and were told “$10 or $15.’’