The Union of European Clubs (UEC) said some 1,400 professional clubs were not taking part in European competitions and had no voice in decisions taken by governing body UEFA that still had a bearing on them.
Representatives of 40 clubs from 25 countries attended, including England's Aston Villa and Brighton & Hove Albion, Spain's Sevilla and Valencia and Germany's Borussia Moenchengladbach.
The UEC will accept membership applications from professional clubs playing in top two national leagues, seeing 200 members as "reasonable to target" by the end of the year. Membership is free.
A challenge the group faces is that the rival European Club Association (ECA) is now the only club body UEFA recognises.
The ECA represents more than 200 clubs across the continent although it is dominated by the big teams, including some of the 12 initially involved in a planned breakaway Super League.
Crystal Palace chairman Steve Parish, a speaker at the Brussels event, said he did not feel his club was represented anywhere at the European level.
Javier Tebas, president of Spain's LaLiga, which backs the UEC venture, said ECA only represented the elite.
Other speakers included Alex Muzio, president of Belgium's Union Saint-Gilloise, who reached the Europa League quarter-finals two seasons after playing in Belgium's second tier.
"I fear we're becoming something of an exception, a rarity. Big clubs are getting bigger and small clubs getting smaller," he said.
The UEC said access to European competition should remain based on domestic league results, with more balanced revenue sharing. Its backers also question why UEFA gives more prize money to clubs that have done better in the past than to equal-performing newcomers.
