Howarth, making his debut at the event, crashed at the Bray Hill section of the mountain course, bringing out the red flags to halt the senior race.
Although the 29-year-old British rider was not hurt, the Isle of Man TT official Twitter feed reported that several spectators were hurt as a result of the crash.
Race organisers announced on Twitter that 11 spectators had been caught up in the first-lap incident but none of the injuries is believed to be life-threatening.
Many of the injured were taken to the nearby Nobles Hospital.
With the riders being taken back to the paddock and the track at Bray Hill needing to be checked over, the organisers announced the race would not restart before 3:45pm.
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