LISBON, Nov 11 (Reuters) - Portugal's premier Antonio Costa, who resigned this week, told foreign investors on Saturday the country was open for business and wanted to remain attractive despite an ongoing corruption probe into "green" energy projects.

Costa stepped down on Tuesday over an investigation into alleged illegalities in his government's handling of lithium and hydrogen projects, as well as a large-scale data centre. President Marcelo Rebelo de Sousa has called an election for March 10. Costa denies wrongdoing.

The data centre project, Start Campus, has been dubbed "one of the biggest foreign direct investments in Portugal in recent decades". The CEO of the project, Afonso Salema, was among those detained as part of the ongoing investigation. He denies wrongdoing.

Alleged government interference in plans by London-based Savannah Resources to build four open-pit lithium mines in northern Portugal is also being looked into. The company said it was cooperating with authorities but that neither the company nor any of its staff were the targets of the investigation.

Lithium projects have faced strong opposition from local residents and environmentalists. They say the processes lacked transparency and have warned of "dangerous promiscuity" between decision makers and mining companies.

Afraid recent developments could have an impact on foreign investment in the country, Costa made a televised address to calm investors, telling them the country's authorities complied with the rules and that there were strong regulations in place.

"To all those who have placed their trust in investing in Portugal, I want to say that today, and always, business investment is desired, welcome and will be well received," Costa said.

He added that "eliminating bureaucracy... in strict respect of the law" was one of his government's priorities to implement projects in the national interest.

Another government official named as a formal suspect in the investigation was Costa's infrastructure minister, Joao Galamba, who previously served as energy secretary. Costa will meet the president on Tuesday to discuss Galamba's future.

Costa's chief of staff Vitor Escaria was also detained. Authorities found nearly 76,000 euros in cash hidden in his office, located in the premier's official residence. His lawyer said the cash wasn't illegal.

"The seizure of envelopes with money in the office of a person I chose hurts me... embarrasses me before the Portuguese and I have a duty to apologise," Costa said. (Reporting by Catarina Demony; Editing by David Gregorio)