The Netherlands has raised its terror threat level, citing concerns that Dutch citizens who have fought in Syria are returning more radicalised.
The authorities have put the level up from "limited" to "substantial" - the second highest category.
The government says there are also signs of growing radicalisation among young Dutch at home.
Security is expected to be a priority in two months when Crown Prince Willem-Alexander officially becomes king.
Hundreds of thousands of people are thought likely to arrive in Amsterdam for ceremonies marking his investiture and the abdication of Queen Beatrix.
"The chance of an attack in the Netherlands or against Dutch interests abroad has risen," said National Co-ordinator for Security and Counter-terrorism Dick Shoof in a statement.
"These jihadist travellers can return to the Netherlands highly radicalised, traumatised and with a strong desire to commit violence, thus posing a significant threat to this country."
Mr Schoof said nearly 100 people had travelled from the Netherlands to Africa and the Middle East to fight in conflicts - many more from Europe as a whole. He said those who had returned to the Netherlands were being monitored.
It is the first time since 2009 that the terror threat in the Netherlands has been raised to "substantial".