An Interior Ministry statement said the activities of the eight could constitute offences that "compromise Spain's peace or independence on being Spaniards who, taking part in an armed conflict, violate the neutrality Spain must keep in relation to the international community."
They are also suspected of being accomplices in killings and of possessing arms, the ministry said.
If found guilty the defendants may face prison terms ranging from four years to 15, The Associated Press reports.
The eight were arrested in early morning raids in six different parts of Spain.
They went to Ukraine last year where they joined pro-independence militia fighting in the country’s eastern Donetsk and Luhansk regions and returned recently, the statement added.
The eight were arrested in early morning raids in six different parts of Spain.
The ministry said the suspects had posted videos on social networks of themselves training, handling arms and expressing support for the pro-independence fighters in Ukraine.
