Rabbi Yitzchak Blazer, Reb Itzele Peterburger, was born on 1 Adar 5597 (1837) in Šnipiškės, near Vilna. His gifts showed early: at fourteen his father printed a sharply argued booklet he had written on tractate Bava Kamma. He studied as a chavruta of Rabbi Naftali Amsterdam.

Married at fifteen, he moved to Kovno and entered the beit midrash of Rabbi Yisrael Salanter, founder of the Mussar movement. He became one of the elite disciples whom his teacher guided personally; Rabbi Yisrael called him "the scholar" among his three great students. Wishing to live from the work of his hands, he learned dyeing, but his teacher forbade it and instructed him to enter the rabbinate.

In 1862, at twenty-five, he became rabbi of St Petersburg, then Russia's capital, almost empty of religious life and a centre of the Haskalah. He strengthened the community and waged an open battle against the leaders of the maskilim, who attacked him in their newspapers. From 1880 to about 1891 he headed the Kovno Kollel founded by his teacher, which became a bastion of Mussar, and was one of the spiritual guides of the Lithuanian yeshivot.

He published Or Yisrael, a collection of Rabbi Yisrael Salanter's letters and teachings together with his own writings, and a volume of responsa, Pri Yitzchak. He spent his last years in the Land of Israel, passed away there on 11 Av 5667 (1907) and rests on the Mount of Olives.