How can I hand-sew an "invisible" hem?
Yes, what you need is called a blind stitch to create a blind hem&hellip; and it can definitely be sewn by hand.

Of course, hemming pants while you are wearing them is a bit more difficult, and it will never look quite as finished without an iron. But in an emergency, this will work great and the stitches will remain virtually unseen.

Start by folding your cuff under to the correct length (wrong-side to wrong-side) and crease the fold as best you can with your fingers. Now grab the entire cuff (two layers of fabric) and fold the hem back to the right side so it leaves just a little bit of overhang (~1/4" or so). 



Now sew along your hemline through the three layers so the stitches will catch only a very small piece on each side of the fold. Run the stitches around the entire circumference of the cuff.

 


When you turn the hem right-side out, you will have a virtually invisible seam.



For an emergency quick tack, you probably wont need so many stitches. The tighter stitches assure you won't catch a toe and rip the stitches out when you put the pants on, but in the situation you are describing, one stitch every inch or so (or even every few inches) should be enough to get you through the day.