The priest in purple vestments moved deftly among the kneeling worshipers who crowded the chapel, placing with his thumb the traditional Ash Wednesday mark on each bowed forehead.

"Remember, O man, that thou art dust and unto dust shalt thou return," the Rev. Frederic Meisel, rector of the Ascension & St. Agnes Church, intoned at the noon mass yesterday the first day of Lent.

Christians of differing traditions observe Lent in different ways. For Roman Catholics, most Episcopalians and some Lutherans, it begins with the special Ash Wednesday services in which the faithful are marked with the ashes, derived traditionally from the burned palms of the previous year's Palm Sunday.

Lent, a 40-day penitential period leading up to Easter, is a time calling for intensified religious devotion, study, prayer, and for some, fasting.

The ashes, Father Meisel reminded worshipers "are not meant to indicate the fasting (of Lent) but our destiny with the Lord jesus Christ."

roman Catholics and high-church Episcopalians, such as those in Father Meisel's parish, still fast during Lent. For Catholics, however, the requirement to abstain from meat except for one meal a day for the entire 40 days has been modified. Current regulations require abstinence form meat only on Ash Wednesday and the Good Friday. Abstinence is encouraged on all other days.

Once-a-week lenten study series on the Bible, prayer or some aspect of Christian doctrine or life have become standard fixtures of most Protestant congregations. So are special musical, or dramatic offerings.

Increased emphasis on charity has also come to be an essential element of lenten observance for most Christians.

Roman Catholics nationwide are participating, as a part of lenten observance, in "Operation Rice Bowl." Sponsored by Catholic Relief Services, the campaign encourages Catholic families to skimp on at least one meal a week during Lent and contribute the money saved to a special church fund for the relief of world hunger. About $5 million was collected in this fashion last year.

Protestant and Eastern Orthodox Christians in this country make a comparable effort, coordinated by an agency of the National Council of Churches.

This year, for only the second time in 10 years, Protestant and Roman Catholic churches will celebrate Lent and Easter at the same time as Eastern Orthodox Christians, who follow the Gregorian calendar.

For Eastern Orthodox churches, however, Lent began three days ago with "Clean Monday," when Orthodox faithful are asked to begin a moral and spiritual purification through self-examination, prayer and fasting.