DAR ES SALAAM, Tanzania — Tanzanian security forces have detained dozens of opposition-aligned activists in the lead-up to a scheduled series of anti-government demonstrations, launching a preemptive crackdown on growing calls for sweeping democratic changes and the release of jailed opposition leader Tundu Lissu. The planned protests, called by youth activist groups for Tuesday, are intentionally timed to coincide with two major events in the country: the 72nd founding anniversary of Tanzania’s long-ruling political party, and the opening of the 50th annual Dar es Salaam International Trade Fair, widely known locally as SabaSaba — one of the East African nation’s largest recurring commercial gatherings.