Bernie Sanders has now exceeded (as of Oct 10) 250,000 participants in his events from the date of his announcement May 26. He's attended at least 81 public events, a little more than half of which were in Iowa and New Hampshire. Most of those were fairly small and hard to get accurate counts for. However, the top 25 events alone account for 224,900 per published counts (and including overflow) and total of the events I can find decent numbers for = 237,700. Assuming 300 average at the smaller events that I wasn't able to get numbers for - events which range from meetings in homes to VFW halls to HS auditorium-size venues) (not a bad average estimate, I think, and even if it's a hundred or more off it doesn't affect the numbers by by a few thousand), this totals out to just over 250,000.
This is far, far more than any other candidate in either party has drawn so far.
Top 25 Sanders events (unfortunately the neat table I make in the edit mode, doesn't work when displayed, sorry for the slop):
9-Aug Portland, OR 28000
10-Aug Los Angeles, CA 27000
3-Oct Boston, MA 24000
8-Aug Seattle, WA 15000
14-Sep Manassas, VA 12000
18-Jul Phoenix, AZ 11000
9-Oct Tucson, AZ 11000
1-Jul Madison, WI 10000
13-Sep Greensboro, SC 9000
10-Oct Boulder, CO 9000
6-Jul Portland, ME 8000
19-Jul Dallas, TX 8000
3-Oct Springfield, MA 6000
26-May Burlington, VT 5000
20-Jun Denver, CO 5000
19-Jul Houston, TX 5000
26-Jul Kenner, LA 4500
18-Aug Reno, NV 4500
19-Sep Manchester, NH 4200
31-May Minneapolis, MN 4000
22-Aug Charleston, SC 3100
12-Sep Rock Hill, SC 3000
20-Sep Durham, NH 3000
21-Aug Greenville, SC 2800
21-Aug Columbia, SC 2800