The group is suspected of illegally importing animals from Eastern Europe in small, poorly ventilated vehicles over distances longer than 2,000 kilometers, the police said in a statement.
The unsanitary conditions caused some of the animals to develop diseases. Some died because of the neglect.
Authorities suspect the group falsified documents to pass sick animals off as healthy when they were sold.
Police launched the investigation in late 2020 after receiving several complaints about unsanitary conditions at a pet store in central Barcelona.
The police operation against the network, where the hundreds of animals were saved, was carried out last month.