Heavily armed Russian troops guarded subways, trolleys and bus stations against a feared wave of terrorism Friday after a bus bomb shattered Moscow’s morning rush hour for the second straight day.

About 1,000 Interior Ministry soldiers were called out to beef up city patrols and police were put on round-the-clock full alert.

Eight of the 28 people injured in Friday’s explosion were reported in serious condition, and residents riding public transportation later in the day were noticeably on edge.

No public claim of responsibility has been made for this week’s bombings. Official suspicion fell on Chechen separatist rebels.