We actually had a massive police incident two weeks ago (a burglar evading the police, big roped off area, lots of activity). So it's pretty much business as usual. For the most part people are just standing about on the Green. I think unless you were on the actual tube it's hard to really know the extent. Massive police presence but people seem calm and just chatting.
My brother was on tube that got stopped at Putney Bridge too.
Someone just said that an eye witness saw a fireball. I'm guessing not huge, but meant that it was some kind of incendiary. Lots of reporters.