Tusk wrote on the X platform that "the worst suspicions" had been confirmed after an explosion destroyed a section of track near the village of Mika on the Warsaw-Lublin route.
He said emergency services and prosecutors were working at the site.
"The blowing up of the railway track on the Warsaw-Lublin route is an unprecedented act of sabotage aimed at the security of the Polish state and its citizens," Tusk wrote.
He added: "An investigation is ongoing. As in previous cases of this type, we will catch the perpetrators, regardless of who commissioned them."
The prime minister added that damage to tracks had also been found on the same line closer to Lublin. He gave no further details.
Interior Minister Marcin Kierwiński later said there was "no doubt" that the incident at Mika was an act of sabotage, while another section of the strategic route was being examined.
A train driver reported irregularities on the Dęblin-Warsaw route early on Sunday near the village of Życzyn, about 100 km from the Polish capital, according to police in the central city of Radom.
Initial inspections showed part of the track had been damaged, police said.
In a separate incident on Sunday night, Lublin police said a passenger train from Świnoujście to Rzeszów carrying 475 people made a sudden stop after 9 p.m. local time.
No one was injured, but windows in one of the cars were shattered, most likely by a damaged overhead power line, police said.
Officers working under the supervision of the district prosecutor's office in Puławy were at the scene along with officers from the Lublin provincial police headquarters, the Central Bureau of Police Investigation and the regional delegation of the Internal Security Agency (ABW), the Lublin police said on X.
In response to the incidents, ministers and security chiefs met at the Interior Ministry on Monday morning.
The meeting was being attended by Kierwiński, Justice Minister and Prosecutor General Waldemar Żurek, special services coordinator Tomasz Siemoniak and the heads of agencies responsible for state security.
Tusk, Kierwiński, Siemoniak and national police chief General Inspector Marek Boroń visited the site of the blast on the Warsaw-Lublin line on Monday morning.
Siemoniak said the ABW was working there with police, prosecutors and rail authorities, and that the latest findings had been presented to Tusk.
Deputy Prime Minister and Defense Minister Władysław Kosiniak-Kamysz said military units were also supporting the investigation near Garwolin and would check around 120 km of railway corridor toward the Ukrainian border at Hrubieszów.
Territorial defense forces, including the 2nd Lublin Brigade and its 25th Light Infantry Battalion from Zamość, were involved, he said.
The National Security Bureau (BBN) said President Karol Nawrocki was being kept informed and that it was gathering and verifying information on what it described as deliberate damage bearing the hallmarks of sabotage on line No. 7, which runs from Warsaw to the Polish-Ukrainian border crossing at Dorohusk.
(jh/gs)
Source: Polskie Radio 24, PAP