Those who are in custody are helping police with their ongoing inquiries and some are expected to be released on bail, Detective Inspector Ross McKay said.
He also said some would remain in custody as potential charges are confirmed.
The main search area was adjacent to a local primary school and police used the opportunity to speak with the pupils there and offer reassurance.
"The Whanganui community can expect to continue to see an increased number of armed police, investigative staff, and the Eagle helicopter in the area as we work on the investigation," McKay said.
Earlier today there was more than 10 police vehicles in attendance and four or five people are in handcuffs being spoken to by officers.
A number of those being detained were wearing the colours of the Black Power gang, known to have a strong presence in Castlecliff.
CIB detectives also attended the scene.
Ratana, 27, was shot dead in Puriri St, Castlecliff about 9.40am on Tuesday morning, amid reports of a gang clash in the suburb.
The offenders remain on the run, however, it was believed those involved were known to each other and were known to police.
