This looks like either a young or rather hungry Theridiid spider. I am not particularly experienced with Eurasian spiders, but as far as I know, the only Widows you're likely to come across in Karachi are either the Redback (Latrodectus hasselti) or the relatively non-problematic Brown Widow (Latrodectus geometricus). This does not appear to be either of these - the front pair of legs is nice and long, but the abdomen is too plain and the bands at the leg joints are not particularly dark brown and contrasting.
Young female Brown Widow from Bug Guide:
Male Widows, both Black and Brown, tend to have small, rather stripy abdomens, and as adults, will have enlarged ball-like structures at the end of the face-mounted pedipalps.
Male Black Widow from Bug Guide:
That just leaves one of the medically unimportant Theridiids as the most likely candidate, and that's where I unfortunately cannot help any further. Still, the rule of thumb is that unless you identify it as a problem biter, it's not worth worrying about. The trick is to look at its underside. If it shows a red hourglass, it's a problem - if not, no worries.
Judging by the photos, I'd go with "no worries", unless you can come up with a clear image of something troublesome.
I include references to support my claims regarding the distribution of Widows in Pakistan: