Thanasis Antetokounmpo expressed his pain over the bloody accident in Tempi with the train collision through his post on Instagram.
The Milwaukee Bucks player posted a heart next to the Greek flag, writing that "in the darkest days, hope and love will remain. Although sadness may take a long time to leave our hearts, hope and love will never leave," wrote the Greek National and NBA star.
Thanasis Antetokounmpo's post:
“Grief can be hard to bear, But hope and love can still be found there. Though sadness may linger in the air, Hope and love will always be there. In times of sorrow and despair, Hope and love will still be there.
"Though tears may fall and hearts may ache, Hope and love will never break. In the darkest of days, Hope and love will still remain. Though grief may linger in our hearts, Hope and love will never part."
CCTV captures the moment of the fatal train collision
Rioters clashed with police outside the headquarters of Hellenic Train in Athens - the company responsible for maintaining Greece's railways.
Protests were also held in Thessaloniki and the city of Larissa, near where the disaster happened on Tuesday night.
The government has said an independent investigation will deliver justice.
Three days of national mourning have been declared across the country following the incident, in which a passenger service crashed head on into a freight train, causing the front carriages to burst into flames.
The front carriages of the passenger train were mostly destroyed.
Many of the 350 passengers on board were students in their 20s returning to Thessaloniki after a long weekend celebrating Greek Orthodox Lent.
Prime Minister Kyriakos Mitsotakis said "tragic human error" was to blame for the disaster.
A 59-year-old station master in Larissa has been charged with manslaughter by negligence. He has denied any wrongdoing, blaming the crash on a technical fault.
Rail union members believe safety systems were not working properly, with repeated warnings about this over many years.
In protest and mourning, rail workers are planning on striking on Thursday at what they say is official neglect of the railways.
"Pain has turned into anger for the dozens of dead and wounded colleagues and fellow citizens," the workers' union said in a statement announcing the strike.
"The disrespect shown over the years by governments to the Greek railways led to the tragic result," it added in comments cited by Reuters news agency.
Transport Minister Kostas Karamanlis resigned over the disaster, saying he would take responsibility for the authorities' "long-standing failures" to fix a railway system he said was not fit for the 21st Century.
But outside a hospital where bodies of the train crash's victims were being brought, a banner was hung claiming that any systemic failings would be covered up in the official investigation now under way.
At a silent vigil in Larissa on Wednesday to commemorate the victims of the incident, one demonstrator said he felt the disaster had only been a matter of time.
"The rail network looked problematic, with worn down, badly paid staff," Nikos Savva, a medical student from Cyprus, told AFP news agency.
The station master arrested should not pay the price "for a whole ailing system", he added.
"This is an inadmissible accident. We've known this situation for 30 years," a doctor based in Larissa, Costas Bargiotas, told AFP.
A vigil was also held in Athens, outside the offices of Hellenic Train. Later in the day, things turned violent in the same area, with police using tear gas to disperse protesters who threw stones and lit fires in the streets.
READ MORE: Protestors and riot police clash outside Hellenic Train headquarters after horrific collision death toll reaches 43.