BEIJING (REUTERS) - China has appointed former Communist Party secretary of Hubei province Li Hongzhong as the new party chief for the strategic port city of Tianjin, days after announcing a corruption investigation into the northeastern city's former mayor.

Dozens of senior officials have been investigated or jailed since President Xi Jinping assumed power almost four years ago, vowing to go after corruption and warning, like others before, the problem threatens the party's grip on power.

The party's graft-busting Central Commission for Discipline Inspection said on Saturday (Sept 10) that the mayor and acting party chief of Tianjin, 61-year-old Huang Xingguo, was suspected of"serious discipline breaches", a euphemism for corruption.

"Huang Xingguo will no longer act as Tianjin party secretary, has been excused from his positions as municipal deputy party secretary, standing committee member, and mayor, and will be handled according to relevant laws and regulations,"Xinhua said. It gave no other details.

Li, 60, has been appointed as Tianjin's party secretary, a post that outranks the mayor, and would no longer hold his positions in Hubei, the official Xinhua news agency said in a short article.

Tianjin is an important port city about an hour from Beijing by car that has ambitions to become a financial hub for northern China. It is one of four conurbations - along with Beijing, Shanghai and Chongqing - termed a municipality, giving it the same high status as a province.

In August last year, a series of massive explosions at a chemicals warehouse in Tianjin killed about 170 people, sparking anger nationwide that it had been built so close to residential areas.

Last month, the party announced it was investigating Yin Hailin, a long-time city planning official who became Tianjin's deputy mayor in 2012, also on suspicion of corruption.

Huang's fall from grace appears to have come out of the blue. On Saturday, the official Tianjin Daily covered his visit to a middle school on its front page and praised teachers for their work.

Critics have accused Xi of using the corruption campaign as a cover to root out political rivals, though he has denied this.

Last year, a Chinese court jailed former domestic security chief Zhou Yongkang for life for bribery, leaking state secrets and abuse of power.

He was the most senior Chinese official to be ensnared in a graft probe since the party swept to power in 1949.