Ebullient Estonians, Wary Soviets. REPORTER'S NOTEBOOK

WHAT a different welcome. On my first visit five years ago to Tallinn, I came from prosperous, smart Helsinki. In comparison, this old Hanseatic seaport was a collection of mud-covered streets and crumbling facades. It was hard to strike up a conversation with gaunt, reserved Estonians.

This time, I arrived from Moscow, and Tallinn looked like a return to European civilization. The cobbled, if still mud-covered streets, evoked an Old-World charm missing from the concrete muddle of the Soviet capital. The stores still are somber, poorly lit, and dirty, but there are some new cooperatives selling T-shirts reading in English, ``Glasnost is Hot News!''

Best of all, the placid Estonian has been replaced by a voluble, new breed. At the headquarters of the Popular Front - the new nationalist mass organization - Eino Muriste shows the way to a seat with the exclamation, ``American journalist, good, very good. Let's talk.''

His mood is excited, upbeat. Just a week before, the Estonian blue, black, and white tricolor had been raised on the Pikk Herman tower. Ever since the Baltic state was occupied by Soviet troops in 1940, people have been imprisoned for flying that flag.

``We used to be so scared,'' he says. ``We are a large movement. We can push perestroika in the right direction.''

An activist enters the office. ``I know some good people to talk to,'' he says. ``Let's go.''

We climb winding roads to the Old Town's main square. There we meet Helje Kaskel, the organizer of the First World Festival of Young Estonians. ``We used to consider people who emigrated from here traitors,'' she explains. ``So we're arranging a festival for traitors.''

Her infectious, if sometimes ironic, enthusiasm also is found at the Homeland Magazine. Editor Tarmu Tammerk says his former censor has disappeared.

``We have no more taboos,'' he says, pausing for a second to reflect. ``Perhaps you have to be a little careful about writing on Gorbachev - but then again, we've been criticizing him, too.''

For many of the local Russians, who have come to man new factories, this taboo-breaking is frightening. At a local pub, they explain why they have formed an anti-Estonian group to preserve Soviet unity called, with a certain amount of sardonic wit, ``the internationalists.'' To them, the Estonian Flag is still blasphemy.

``OK, so we occupied this place,'' says Ivan. ``It's a long time ago.''

Somber talk can also be heard among Estonians. Rein Magar, a founder of the Heritage Group demanding immediate independence from the Soviet Union, outlines his plans for the unwelcome Russians.

``Those who learn Estonian, those who recognize the Estonian state, they can stay,'' he says. ``The others can go.''