* President says charges against aide, others "very serious"
* Court holds some detainees in custody, to decide on others later
* Charges part of wider graft investigation
* PM's coalition partners considering next steps
By Jan Korselt
OSTRAVA, Czech Republic, June 15 (Reuters) - The coalition partners of Czech Prime Minister Petr Necas said they were considering whether they could stay in government with him on Saturday after a court ordered the detention of a close aide to Necas over corruption charges.
A court in the eastern Czech city of Ostrava ruled that Jana Nagyova, who has been in charge of Necas's office for years, be remanded in custody. Prosecutors allege she bribed politicians and illegally ordered intelligence agents to conduct surveillance operations.
After the court ruling, an official with TOP09, the bigger of two parties that are in coalition with Necas, said party leaders would meet on Saturday evening to decide what to do about staying in the coalition.
Karolina Peake, leader of the second junior partner in the coalition, the small liberal party called LIDEM, told Reuters: "The situation is becoming more serious from hour to hour."
Necas's Civic Democrat party alone does not have enough seats in parliament to hold on to power, so if either of the junior partners turn against the coalition, he would fall. That would lead to either a new election or president Milos Zeman could try to pick a new prime minister to form a new cabinet.
The government has been in turmoil since prosecutors charged Nagyova and seven other people as part of the biggest sweep against suspected political corruption in two decades.
Starting around midnight on Wednesday, around 400 officers, some clad in balaclavas to conceal their identity, raided 31 premises, including bank safe deposit boxes, and seized at least $6 million in cash and tens of kilograms of gold. Prosecutors said more charges may follow, but declined to give details.
The court in Ostrava did not rule on the substance of the charges, but by keeping Nagyova in jail it showed it believed prosecutors at least had a credible case. That made it harder for the governing coalition to dismiss the allegations as a witch-hunt by rogue prosecutors.
Nagyova's role is crucial to the prime minister's political survival because, even though there are no allegations he was involved, the two are known to have worked very closely together for years.
In a speech to lawmakers on Friday, the prime minister dismissed the allegations and said he would stay on. He said he had done nothing dishonest.
Speaking outside the court in Ostrava, Eduard Bruna, a lawyer for Nagyova, said she rejected some of the accusations against her and was arguing that she had acted in good faith on others.
Earlier on Saturday, Czech President Milos Zeman was asked by reporters whether he thought the centre-right cabinet led by Necas should stay in office.
"I consider the charges that have been brought to be very serious," sad Necas, who is a political opponent of the prime minister.
"After hearing from the chief of police and the supreme state attorney, I am coming to the conclusion that they are based on sufficient evidence," he said in his first remarks since a series of police raids on government offices this week.
"This is an indirect but clear answer to your question," he said at a ceremony north of the capital to commemorate Czech victims of the Nazi occupation.