A police spokesperson said the attack occurred in the Kuecknitz district of the city and a suspect has been apprehended and arrested.
Police from Schleswig-Holstein said on Twitter that "people were injured".
"There is now a major police deployment in Luebeck".
Contacted by Reuters, a police spokesman declined to give any details about the attacker or number of people injured, but said police were examining a backpack he may have been carrying. Authorities have not yet confirmed the suspect's motive, but prosecutors said they had not ruled out terrorism.
Hingst also said police are investigating a suspicious rucksack after witnesses reported that the man had been carrying a "smouldering" bag on the bus headed for the Baltic coast. "For that reason we have, as the investigation now stands, no indications that there was a terrorist background to this act". Police said authorities were still trying to determine the circumstances of and motive for the attack. As the suspect left the bus, he stabbed people around him with a 13-centimeter (5-inch) kitchen knife.
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"According to local reports, witnesses say the bus was packed with passengers when a man dropped a "smoldering backpack" and started attacking people".
Eight people have been hurt in an attack on a bus in the northern German city of Luebeck.
He was shot dead by Italian police in Milan four days later while on the run.
Germans have been on edge since December 2016, when a Tunisian whose request for asylum was turned down and who had links to Islamist militants hijacked a truck and ploughed into a crowded marketplace in Berlin.