MADRID (REUTERS) - Spanish lender Santander evacuated the main building at its Madrid headquarters and issued a security alert on Tuesday after envelopes containing white powder were sent to senior executives.

Santander, Europe's biggest bank by market value, said it had called in Spain's Guardia Civil police and its specialist unit dealing with chemical threats as a precaution, and was sending off the envelopes to be analysed.

The 12 employees who handled the envelopes were being kept under medical observation, though none were suffering from any unusual symptoms, the bank said in a statement.

Some of those who came into contact with the envelopes were secretaries, a source close to the bank said, adding that the main building in Santander's sprawling financial centre on the outskirts of the Spanish capital was evacuated.

"With the exception of the affected offices, the rest of the central services of the bank continued to operate normally,"Santander said.

Santander did not specify which executives the envelopes had been addressed to, only saying they were for members of its management team.

Such incidents are not unheard of in Spain, though recent alerts have not involved banks.

Last year, Amnesty International evacuated its offices in Madrid after receiving a suspicious envelope, while this year several media companies called in police after similar packages were received.

None were confirmed to be serious threats.

The envelopes sent to Santander were delivered to the building housing the offices of its top staff, including that of chairwoman Ana Botin, the source said.

The spaceship-like building sits in the middle of the bank's vast campus, which as well as offices and conference rooms has sports grounds and many restaurants.

It is Santander's main headquarters for corporate activities, though the bank is incorporated in the port city of Santander, in northern Spain.