By Maarten van Tartwijk and Matina Stevis 

AMSTERDAM--Flags were flying at half-mast across the Netherlands on Friday, as the nation began a period of mourning after one of the deadliest plane crashes in Dutch history.

The Netherlands was slowly digesting news of Thursday's crash of Malaysia Airlines Flight 17 , with many citizens and officials expressing disbelief, grief and anger.

Ivo Opstelten, the Dutch minister of security and justice, said the number of Dutch victims had been revised upward to at least 173 casualties. "The terrible reality of this disaster is slowly coming in," he told a news conference.

Mr. Opstelten said the cause of the crash was still unknown, but that it was most likely that the plane was "shot down." He said the Netherlands should be part of the international team that will investigate the crash site, and that he hoped to provide more information about the probe later on Friday.

Relatives of the passengers on board of Thursday's fatal flight were staying at a hotel near Amsterdam's Schiphol airport Friday. The hotel was guarded by police on explicit orders to keep the press off the premises. One officer said family members had been trickling in Friday.

The fatal flight carried several Dutch citizens who were on their way to an AIDS conference in Melbourne. One of them was Joep Lange, a globally recognized AIDS activist who formerly served as president of the International AIDS Society.

Another well-known person on board was Dutch Senator Willem Witteveen, the Dutch Senate said in a statement on its website.

Schools posted obituaries on their Facebook accounts, to honor students and teachers that were on the plane. Families and friends also paid tributes to the victims via social media.

Cor Schilder and his girlfriend Neeltje Tol, who were on the flight, were going on vacation. Mr. Schilder, whom friends described as outgoing and adventurous, took a picture of Flight 17 and posted it on his Facebook page. His message read: "Should it disappear, this is what it looks like."

The couple, who owned a flower store in Volendam, north of Amsterdam, had left a message on the store's window. "We are open again on August 4 from 12:00. We wish everyone a very pleasant holiday."

In the meantime, at Schiphol Airport, passengers said they were afraid to board another Malaysia Airlines flight to Kuala Lumpur, which was set to depart on Friday afternoon.

"I don't want to go," Angela Molina told reporters, while her eight-year-old son Tristan was hiding behind her. Ms. Molina, from Melbourne, Australia, was visibly shaken. She said she had been in touch with her travel agent but had not been given the option to cancel her flight and move to another airline.

Dozens of passengers were hurriedly ushered through to their gate Friday morning by airport staff as reporters and camera crews flocked here.

The Malaysia Airlines counter at the Dutch airport was unstaffed.

Write to Maarten van Tartwijk at maarten.vantartwijk@wsj.com and Matina Stevis at matina.stevis@wsj.com

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