A couple of years ago, Lewis Jay Meyers, a co-founder of the South by Southwest Music Conference and Festival in Austin, Texas, stood in the middle of Austin’s blocked-off Sixth Street, the heart of the club district, as SXSW attendees poured down the sidewalks and traffic lanes.

Meyers shook his head in amused disbelief and said, “You know, this isn’t quite what I envisioned when we started this.”

This year, SXSW’s music component, which kicks off Tuesday, marks a quarter-century as music’s springtime nexus for networking, schmoozing, scouting and rocking out.

Organized by Meyers (now director of Memphis’ Folk Alliance), Austin talent manager Roland Swenson (today SXSW’s executive director), Louis Black and Nick Barbaro, editor and publisher of the alt-weekly the Austin Chronicle, the first SXSW was a humble affair. The 1987 event’s panel sessions and its 700 registrants were housed in one downtown hotel, and just 172 bands comprised the official bill at 13 venues.

This year’s 25th SXSW Music will attract nearly 12,000 registrants. The conference will take over the 881,000-square-foot Austin Convention Center for five days. The official 2011 roster of 2,000 acts will perform in 90 venues; hundreds of others will play guerrilla-style unofficial shows. And every hotel in the Austin area is booked solid.

Not bad for an event that was founded in 1987 as a regional equivalent of New York’s New Music Seminar, then America’s preeminent music conference. Swenson, who began attending NMS in 1982, thought a similarly styled, club-hopping event could benefit the Southwestern music industry.

“If you lived in Austin, you didn’t have access to the industry people in the major cities,” Swenson says. “Part of the idea was to bring bands and club people and label people from the cities in our area together in one place — that it would create this bigger market for all the artists to use.”

The first SXSW managed to make money, and instantly attracted the interest of major label A&R scouts. It drew people by consciously avoiding any appearance of provinciality or insularity.

“We didn’t want to call it the Austin Music Hoo-Ha,” Swenson says. “We knew if we didn’t get people from outside Austin to come to it, it wouldn’t be a success. And we didn’t want to tie it down to any style of music.”

The founders also actively romanced potential international players: Meyers beat the bushes in Germany in 1988, and Swenson traveled to MIDEM in Cannes in 1989. The early legwork paid off: Today, some 30 international music trade orgs participate.

The mid-’90s saw SXSW’s detonation as a national event. It moved into the new Convention Center in 1994, the same year film and interactive programming were launched. In 1995, NMS folded, and Austin became the locus for the majors’ promotional activities.

Swenson ties SXSW’s ability to weather the last decade’s precipitous downturn in the music industry to the original conference’s roots: “We’ve always had a broad constituency of people.”