His assassination by right-wing zealots that were covertly supported by the police and military provoked mass protests and led to a political crisis.

He also earned several gold medals in the Balkan Games, which took place annually, featuring competitors from Greece, Albania, Yugoslavia, Bulgaria, Romania, and Turkey.

In 1943 he set up the Union of Greek Athletes (Ένωση των Ελλήνων Αθλητών, Enosi ton Ellínon Athlitón) and organized regular competitions.

[3] That same year (1961), under his initiative, the Commission for International Détente and Peace (Eπιτροπή για την Διεθνή Ύφεση και Ειρήνη — Epitropí gia ti Diethní Ýfesi ke Iríni — EDYE) was established in Greece.

In his capacity as Vice President of EDYE, Lambrakis participated in international pacifist meetings and demonstrations despite frequent threats against his life.

EDYE was an organization set up by EDA which while not officially aligned to the World Council of Peace broadly supported its anti-capitalist and anti-imperialist position.

On 22 May 1963, shortly after he had delivered the keynote speech at an anti-war meeting in Thessaloniki, two far-right extremists, Emannouel Emannouilides and Spyros Gotzamanis, driving a three-wheeled vehicle, struck Lambrakis with a club over the head in plain view of a large number of people and (allegedly) some police officers.

The assassination of Lambrakis initiated an enormous popular reaction, and soon after, investigator Christos Sartzetakis, district attorney Nikos Athanasopoulos and attorney general P. Delaportas uncovered connections of the police and army, including the Gendarmerie commander for Northern Greece, Konstantinos Mitsou, and various local police officers, like Kapelonis and Katsoulis, to far-right extremists.

"), a popular graffito which began to appear on the walls of the buildings of the Greek cities in the 1960s, illustrating the growing protest against the conditions that led to the assassination of Lambrakis.

After the fall of the military dictatorship in 1974, numerous places, including a football stadium in Kallithea and streets and squares throughout the country, have been named in honor of Grigoris Lambrakis.