Her son and party vice president Rahul Gandhi also expressed shock at the killing of the firebrand journalist in Bengaluru yesterday and said no one could suppress the truth.
The Congress president described Lankesh as one with fearless and independent views, who had extraordinary grit and determination to take on the system.
"The series of killings of rationalists, free thinkers and journalists in the country has created an atmosphere that dissent, ideological differences and divergence of views can endanger our lives. This cannot be and should not be tolerated," Sonia Gandhi said in a statement.
In condemning this debilitating attack, the Congress party stands as one with the rationalists, thinkers, journalists and the media fraternity, she said.
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Sonia Gandhi has spoken to Karnataka Chief Minister Siddaramaiah and urged him to swiftly bring the culprits to the book.
"Every possible measure should be taken to maintain a safe and secure atmosphere in the state," she added.
Expressing her deepest condolences to Lankesh's family and friends, the Congress president said she offered her prayers.
"No one can suppress the truth. The RSS and BJP ideology is trying to suppress the truth but this cannot happen in India," he told reporters here.
Lankesh, 55, known for her left-leaning outlook and forthright views on Hindutva politics, was shot dead by unidentified assailants at her residence last evening.