"The principle of secularism is the cornerstone of the transformation process" launched by Mustafa Kemal Ataturk on October 29, 1923 when he proclaimed the Turkish Republic on the ashes of the Ottoman Empire, Sezer said.
"Making concessions about the gains and principles of the Turkish Republic is unthinkable," he added.
Many in Turkey worry that the secular system is under threat by growing reactionary movements and accuse Prime Minister Recep Tayyip Erdogan's Islamist-rooted government of seeking to raise the profile of religion in public life.
In a controversial practice, the president has refused to invite veiled women to a traditional October 29 reception in his palace, the top event on Ankara's social calendar, since Erdogan's Justice and Development Party (AKP) came to power in 2002.
Since most ruling party MPs are practising Muslims and their wives wear the headscarf, infuriated AKP members have responded by boycotting the receptions.
The top-selling Hurriyet newspaper reported Saturday that Sezer maintained the same stringent dress code this year and only about 50 of AKP's 355 parliament members were likely to turn up at Sunday's party.
With Erdogan's wife Emine among the women thus snubbed, the prime minister has attended the receptions alone.
In his message, Sezer also warned that Islamic brotherhoods and sects, banned by law but tolerated in practice, "are seeking to expand their sphere of influence, notably in the field of education."
"Their efforts to gain strength and impose authority on citizens... are unacceptable," he said.

Copyright AFP (Agence France-Presse), 2006