"The exact number of injured is still unclear. There were no dead," the police and prosecutor`s office said in a statement.
"The background to the crime is still unclear and the subject of the ongoing investigation," Reuters quoted.
The local Luebecker Nachrichten reported at least 14 people were wounded, two seriously.
On Twitter, the police said the attacker was overpowered and taken into police custody. The attack took place at 1.47 p.m. local time (1147 GMT).
"We ask for your understanding that we cannot provide any information about the identity of the suspect at the moment," the police added on Twitter. "It is important to us to only disclose reliable information in order not to fuel speculation and not to obstruct the investigation."
"The bus was full," the police added, appealing to passengers who fled the scene to contact them.
The Luebecker Nachrichten newspaper cited state prosecutor Ulla Hingst as saying: "Nothing can be ruled out, including a terrorist background."
Luebecker Nachrichten, which said the attacker appeared to be in his mid-30s, quoted a witness as saying: "One of the victims had just given up his place to an older woman, when the attacker stabbed him in the chest."
An area around a bus stop in the Luebeck neighbourhood of Kuecknitz was sealed off by police, spokesman Dierk Duerbook told Bild.
Earlier, police in Schleswig-Holstein, the state where Luebeck is located, tweeted that there was a major police deployment under way in the city.
Luebeck police did not respond to repeated calls for comment.
In April, a man drove a camper van into a group of people sitting outside a restaurant in the German city of Muenster, killing two people before shooting himself dead. There was no evidence of any link to Islamist militancy and he was not a refugee.
In Berlin in December 2016, 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. Twelve people, including the man driving the truck when it was hijacked, were killed.